I am going to post a series of steps for people who have not yet tried double on half. They will be a week apart, to give you time to do the home work.
First week : choose a chair. You can do this with more than one, but roundish is harder than squarish.
Take the cushions off, set it in the middle of the room and observe the topography. With big pins ( so you can see from distance ) and not more than four of them, mark by eyeballing only ( no tape measures ) half way points.
Anyone needing clarification here : double on half is pin fitting method folding cloth at midway point to fit half the chair.
Strategies for finding half way points will vary with the person teaching you, my way is to emphasize eyeballing. Four pins only : top outside back at new slipcover seam , top front deck at curve, bottom outside back at new skirt line and bottom inside back.
The homework is to do this and report back how quickly you were comfortable making a final choice without a tape measure.
You may double check with tape only after you feel you are sure you have the right answer. Importante, use tape in the following manner to double check: measure half, and then the other half. What you are avoiding is trying to divide fractions of inches.
Trusting your eyes, or more exactly, learning for the first time to use and trust your eyes about pin fitting ( learning to separate fresh observations from preconceived notions ) is vital to taking this leap.