I am the SG and I can't ask my players because I'm actually working the rule mechanics for a hedge wizard they'll encounter later in the saga. Hopefully someone will be able to quote the rules I need or provide guidance.

Ars seems to have you round in the direction that is the least advantageous to the characters. So that's the way I'm leaning for the moment, but I'm really not sure how I want to do it.

The way we do it, we always round up unless it's explicitally stated to round down.

Most examples that have division will have (round up) in the forumula (see ArM5 p.225, form bonus, vis extraction, longevity vis cost, spell magnitude, aging total, crisis total) and, in addition, most descriptive text use "and fraction thereof", indicating rounding up.

I think it's consistent enough for us to have chosen rounding up.

Although some specific formulas note "...or fration of", I don't see any reason not to just use normal mathematical conventions, and round numbers below 0,5 down and from 0,5 and more upwards.

I find it inexplicable that everyone hasn't universally acknowledged the superiority of my answer. (humor)

[size=150]I challenge you to find even one example where you need to round.[/size]

The rules for affinities specifically say to round up, they don't count because the rules do tell you how to round.

I challenge because if I'm wrong I'd like to be set right.

My belief is that if you're rounding then you're doing work you don't have to do for the soul benefit of getting less precision. I don't think that it's a wise move.

Well I suppose to be technical you don't HAVE to round, but because the rules generally require a die roll to equal or exceed the target number using fractions where the results are in question is effectively rounding down...

What got me thinging about this issue was when I when one of my players was casting a level 35 ritual. He has Mercurian Magic though and I couldn't decide whether it should him cost 3 or 4 pawns of vis.

I suppose I could have used the actual figure of 3.5, but I don't think it's even possible to have vis in quantities less than a pawn. awaits RoP: M

Well I suppose to be technical you don't HAVE to round, but because the rules generally require a die roll to equal or exceed the target number using fractions where the results are in question is effectively rounding down...

What got me thinging about this issue was when I when one of my players was casting a level 35 ritual. He has Mercurian Magic though and I couldn't decide whether it should him cost 3 or 4 pawns of vis.

I suppose I could have used the actual figure of 3.5, but I don't think it's even possible to have vis in quantities less than a pawn. awaits RoP: M

Mercurian magic says that you halve the vis costs? So for a magnetude 7 spell you'd have a cost of 3.5 pawns of vis.

If you say that one can't use a fractional pawn of vis then the caster has to use four. That comes awfully close to rounding up and meeting my challenge to find a spot where rounding is needed.

[size=75](I was originally going to say something about how a level 17.5 spell is still fourth magnitude but, upon reflection, I realized that this doesn't matter. We're not loking at the vis cost of a level 17.5 spell, were looking at half of the vis cost for a level 35 spell).[/size]

One the one hand, casting sont spells, often results on 0.5 or 0.2 fractions. There are irelevant, so you don't actualÃ¦lÃ¦y have to do the work of rounding up or down, or even wonder which way to round. But you could actually say, that you'd have to round down all the same, to the nearest lower full magnitude. Since all Spell Guidelines are listed as for full magnitudes only. If you see you needa 4th magnitude spell to get the effect you're gunning for, a lvl 17,5 isn't quite that. Tough cookies! So in essence, this situation (and also the aforementioned halving of ritual vis cost from Mercurian Magic), you don't need to formally round up or down. But in practicability you'd do either respectively.

On the other hand, Spont spells needing to Penetrate will be glad for those fractions. If my spell is level 20, Casting Total is 22,5, target's resitance is excately 2, then I Penetrate! Woohoo for me!

Decimals fractions (indeed decimals!) have not been invented yet - try doing the math with proper Roman Numerals and you will see why the rules assume fractions are dealt with.

AFAIK to guiding default rule throughout 5e is "round up" (except where some rule explicitly says "drop fractions" or "round down")

Those aren't realistic examples given the rules, and in general I can't see that you'd do much with a fractional number once you've got it. perhaps subtract a whole number from it in the case of penetration

Just to re-iterate it is easier FOR ME to always round up and then do calculations, than it is to do even the most simple calculations with fractions....