Measuring front-end sag
We're going to tell you how to measure sag in just a minute, but first: It doesn't really matter. That's right. The key to getting a proper front end setup is in getting the right springs and spacers. Slack will take care of itself. Remember, the length of your springs + spacers should be the same as in the stock EX250. The spring rate is what determines how well your front suspension will work. We know how hard this is to accept; deal with it.
Measure the front end topped out and riderless. Then, with help, measure how much the front sags with you and your riding gear on the bike. If you have sag over 1.5", you NEED new springs.
Not new spacers, either (see above and this link). Problems occur when people twist the sag information and think that adjusting the sag when on the WRONG spring (for instance, stockers with 4" of spacer) will improve the bike. Again, the right tool for the job is the way to go. Contact Sonic Springs and take care of the problem correctly.