I got a 2006 KX250 at Christmas. The seat height is was 38" before the sag when you sit on it.
Then with sag it is about 34". It was hard to get on, start, and ride in some corners since my inseam is only 32".
The best fix I found was to modify the seat it has way too much padding in the middle anyway. Most
of the guys I see on TV or in the magazines do not seem to have stock seats. A lot of those guys are shorter than me (5'-9") and their feet touch the ground at the starting line.
I took the seat cover off, marked the foan with a marker to get a design that dipped 2" in the middle,
and then faired up to the existing levels at the front and rear. I got the seat secure so it would not move and then used a 4" electric grinder with a thick grinding wheel (not a thin cutting wheel) and shaped the seat a little at the time until I got it where the marks were. It worked great. I can now touch the ground with both feet when stopped and can start it much easier.
There is one other recommended fix that I may do in addition to my seat mod.That is to add a longer link where your rear shock attaches at the bottom. The link is made by Prolink and it is longer than stock so it changes the angle of the shock and lowers the rear end. Then you may want to see if you went to far down with the forks because that will affect the handling.
this is the best advice on lowering. dont lower a dirtbike