The Yamaha spec is 135mm for oil height on your 426 as well as the '99 250. You can raise height to add bottoming resistance. My revalve setups for customers riding MX are between 90-100mm. I have heard of other tuners spec'ing as high as 65mm. You will have to experiment to find out what works best for each rider. Ask if they are bottoming hard and raise accordingly.
For my 2000 YZ250 forks (probably very nearly the same as the 426's), I tried 100 mm, and got lock about 10-15 cm from bottom. About 125 seems to work for me, and I'm a big guy (220 lbs.). Oil is 7,5 wt.