It is so uncritical I never bother to measure- I only estimate. My guess is my mixes run from about 1:10 to 1:100, paint to thinner. If you are just getting started in washes, I'd go thinner rather than thicker. Thin washes may require several applications, but that allows you to stop when it is correct more easily. You can overdo it with a thicker wash.
I sometimes merely shake a bottle of paint, lay the cap upside down on bench and put a thin (maybe 1/32 to 1/16 inch layer of thinner in cap and start with that. I almost always try a wash first on a painted area not very visible, to test it, but a thin wash can be wiped off without harm if done in 15-20 seconds or so (enamel washes, don't know how soon acrylic washes must be wiped off). Washes are so thin I'll bet you one can't tell the difference over about a 3:1 variation in thinning ratio.