I'm not sure if this is covered in any book (I don't think it's in ArM5). If a magus casts Rego Herbam to move a log through the air, but decides that he and 6 grogs are going to sit on it and travel around, how do troupes/storyguides rule that? Do you add 1 (or more) magnitudes for weight? Do you add a Corpus requisite?
I note that transporting the same group through the air using Rego Corpus would be approximately 2 or 3 magnitudes higher, by comparison.
There is no corpus requisite for something like that. You can, after all, move a ship that happens to have passengers in it, with just rego herbam. Bear in mind that, yes, it might be easier to make a flying log they sit on fly than grant a group of 10 flight than, but that spell is probably less powerful. For one, they can't fly individually - they do it as a group together. The log probably is flying with less control than they could. They could fall off the log and die.
Here's a couple of exemples of spells or items granting flight without corpus form or corpus requisite, some of which animate stuff you ride on:
Look them up, they can give you insight into how to calibrate your player's spell.
Thanks! All of them are interesting, especially the Flight Disk (although I'm doubtful about the validity of the environmental trigger - "this spell is not working" - because if that works surely it would be the specified way to make a constant effect device in ArsM5, p.99. But maybe it's intended to be a legendary exception to what is normally possible with Hermetic magic...?)
I would not necessarily call it a Legendary way to make a constant effect. It is less efficient in levels/vis than the common method (+18 vs +14), while being more resistant to being dispelled. However as written, the way it is controlled should require a second Environmental Trigger. This is seen by the crash caused by sunlight hitting its control point.
An interesting variation of that combination for a different device could be done by changing Duration from Diameter to Concentration and adding "Item Maintains Concentration". This would allow precise control by touching the item and is actually a fairly common way to make a controllable constant effect device.
To return to OP. The chief mechanism that I use to inspire prudence is finesse rolls. The log must be kept steady to ride safely. Generally, I want ReCo variants to be safer, but I have no problem with their being either a higher or a lower level.
Of course, if you make a ship fly, it is much easier to ride than a log or a broomstick, but the ship is larger, so I expect that you pay with a size magnitude.
In this particular case, with seven people on one log, one should think about size, but you need to read (or write) the spell descriptions carefully, because there are different variants. The Unseen Porter, for instance, describes this as a weight/speed trade-off which is not found in the general guidelines.
When the rider and the caster are different people, I might allow the rider an athletics roll to make up for bad finesse. I would not allow that for the caster, because concentrating on the spell leaves little cognitive power to be athletic.
Yes. Magic is NOT designed to be "balanced" in the milquetoast sense of "it doesn't matter how you do it, it's always the same difficulty". There's a reason CrIg is simply better at dealing damage than most other Art-combinations.
So yes, it's easier to fly via some medium (flying carpet/cauldron/rock etc) than by pretending to be superman and flying without any tools. Which is fine by me.
To be honest, that's the main use I've had for Ominous Levitation of the Weighty Stone (HoH: S, p. 38)