Spools and bobbins

The other day I bought one little spool of thread.  I wasn't sure it would be enough, but I thought that I could get two bobbins out of it and still have enough for the top thread.  So I wound my two bobbins.

So yeah, not even close to enough thread left over for the top.  I feel like I do this all the time, so here is my equation, extrapolated from the above picture.  It will help me know how many bobbins I can wind from a spool and still have enough left for the top.  This is mainly for Gutermann; I don't know how it would translate to other types of thread.

Number of bobbins = meters of thread / 100

So in the example above, I had a 100m spool of thread.  I should have just made one bobbin to have a roughly equal amount on the spool and amount on the bobbin(s).  It's even an easy equation to remember!  But we'll see if I actually remember it in the future or not...