When I changed my suspension fork out for a rigid fork, I used this one:

http://www.nycbikes.com/item.php?item_id=713

My original suspension fork had an unloaded axle to crown measure of 450mm, but it compressed down a bit when i sat on it. The new fork measures 420mm, which is pretty close to the original suspension fork's axle to crown measure when I sat on the bike.

If you have a second set of hands available, perhaps you should have someone check the actual axle to crown measurement on your bike while you are sitting on the bike. Your new fork doesn't have to be exactly the same, though it should be pretty close so that your bike's geometry and handling characteristics remain like they are now.