Sorry, but that's just not true. The needs for conversion are few and far between, and to be avoided except in some extreme, unusual circumstance. Most of the world (both the English and metric sides) gets on just fine without having to ever do any conversions. It's been about six months since I took my shop fully metric, to date I have not yet had occasion to calculate a conversion. Sitting here I can't even think of an example of a case where I'd need to calculate a conversion in my furniture making and turning. Perhaps I'd need to do it if I needed to convert published plans, but I don't, as I draw my own. Yes, I have some tools that are marked in even imperial units (eg mortise chisels), but I don't actually care what the measurement is to several decimal places as I fit the tenon to the actual hole I've made in the wood, not to some abstract measurement. I think of that chisel as being ~12.5 mm, that it might actually be 0.5 inches makes no practical difference to my work. I've never measured it to know what its true dimension is-- it could easily actually be 12.7 mm and just marked 1/2".
If you assume plywood or any other pre-dimensioned material is an exact measurement of either sort you will soon be sadly disappointed-- if it matters you'd better put a tape or a caliper on it to find out what it actually is in your favorite units.
The only times I've had to do any conversions in recent times has been in cooking, where it's even more complicated as one needs to weigh stuff to make the shift from recipes given in cups and tablespoons to grams. But, there again, especially in baking, one's outcomes become more reproducible with fewer errors once you have a "metrified" recipe.