KlausC was right about overfilling is worse than underfilling.
But I'd like to add something about checking your fluid level. While it's true that you have two scales (hot and cold) on opposite sides of the dipstick, they are calibrated for temperatures you may not be at when you check. How, in fact, do you know what your ATF's temperature is?
I may sound overly compulsive :-) but I've found -- and here I must point out that I have 240's, and also Mercedes, but not any FWD Volvos -- that a little off with the temperature and you may erroneously think that your fluid level is off when it's not; as well as vice versa.
Let me get to the point... a very handy gadget to have around your shop is a hand-held IR thermometer (like the RayTek unit sold by IPD, Griot's Garage, and many other mail-order vendors). I check the temperatures indicated on the opposite sides of the dipstick, and then I take the transmission's actual temperature by holding the RayTek under the car and up against the sump of the transmission -- that sump temperature should accurately represent the fluid temperature. With that information, I can interpolate the fluid level on the dipstick, at the now known temperature, to where it should be relative to the full and add marks the dipstick. Or in other words, I know with certainty whether I have enough fluid, too much, or too little.
And the weakness in the dipstick scales is that often, the usual remarks about what it takes to warm up the fluid is grossly untrue. For example, the "cold" marks are not the fluid when the car is cold -- and after driving a bit, it may still be higher or lower than you expect. And often, no matter how long you drive, your fluid temperature may not rise to the "hot" temperature -- it really depends on the season (weather), type of driving, etc. I've used my RayTek to prove time and time again that there is no reliable way of warming up your transmission to any certain temperature. So instead, you should measure your fluid's temperature, and then interpolate a third, "mental" scale for your fluid's temperature from the two scales on the dipstick.
Good luck.
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