Not sure what you are getting at, but the actual value ranges do not really matter in an expression. It's more likely you are eitehr having an anchor point issue or your keyframes are simply not linear. in the latter case of course a simple subtraction would not give the correct result because values would be exponential/logarithmic/whatever. You'd have to use much more complex code then to accumulate all values over time such as the great Dan Ebberts explains on his website:
Anyway, I suppose you could always use effects rather than native transforms to scale your matte while still leaving it parented. Y' know, there is a Transform effect... That would be much simpler and also avoid the expression evaluation bogging down your system...
I apprecate your thoughts! I'll definitely check out the Transform Effect, which I always forget about. Is that generally faster than applying expressions directly to attributes?
Simply put... parenting would solve my issue. But, I don't want my two layers parented to eachother permanently. This was something I was hoping I could script via some conditional statement.
The alternative was to right an expression that would acheieve the same Scale matching as well as the parenting does. It wouldn't be a problem if these layers were scaled the same.. but I have one at like 18% and another at 55%... and unfrotunately, just adding 10% to each doesn't, of course, result in the same size increase. Well - let's just say I figured that out after the fact... duh.
Thanks for the link - I actually perused his writeup a bit earlier.