I have an 18 inch radius oval and a 22 inch radius oval that I would like to connect with a left and right crossover.  Is the distance between the tracks on the crossovers just the right size to match the 4 inch difference in these two diameters?  If so, I could use one on each side of the oval and not worry about matching the length of the crossovers to the standard 9 inch straights.

Not much chance of that, I guess, so I have to use the two crossovers on the same side of the oval and make up the odd difference in track length with small straights.  But what is the length of the crossovers?  I've seen 21 inches, 21.25 inches, and 21.5 inches.  If they are 21 inches, then 2 would add up to 42 inches and a 3 inch straight would make 45, which matches 5 nine inch straights.  If that isn't the length, what combination do I need?

thanks