I'm improving my familiar bond by adding effects, but I have stumbled on this issue: If I were to add a CrMe 15 effect and a CrMe 25 effect during the same work season in the familiar bond, would I need 5 Mentem (or Creo) vis or only 4?
The problem of Vis divisibility! Are the numbers when counting vis natural numbers or real numbers? (or rational numbers? Complex numbers!? Quaternions?!?...)
I had read about Sagas that houserule that vis is divisible, so if you have vis source of ten roses that bloom every spring worthing let's say 1 pawns of vis, then each rose's value is 1/10 pawns of vis, and then you could use 2,5 pawns and 1,5 pawns for each enchantment, spending 4 pawns of vis. But I don't like it as I find it too rational and I like magic to be far for optimal managing vis. So I'd say that as by the rules, you would need to round vis up and spend 5 pawns.
Without hesitation I'd calculate vis for several effects during same season as the vis cost had they been performed in different. There is a 'vis loss' for enchanting lower levels than a pawn could potentially cover. Rounding up or down always leads to odd things.
As an extreme example one could lump together 10 effects of lvl 1 and effectively pay 1/10 the normal price of it. For me it's mostly a balance issue, and to avoid undue speculation. I know, you can already speculate with the levels of effects compared to your lab total.
I'm pretty sure that putting powers in a familiar bond is similar enough to instilling enchantments in a greater device that the answer is 5 (2 for the level 15, 3 for the level 25).
Thank you for your answers, it matches what I thought.