This is from "Sailing the Hobie 20 in a Breeze by Bob Curry"

Mast Rake
Start by marking off in 1" increments from behind the rear beam 5 through 10 inches. To check your mast rake: After you have marked off the increments, as I suggested, take the crew's trapeze ring. Detach it from the bungie cord. Tie a line to the ring and take the line forward to the top of the pin that connects the bridle wire to the hull. Pull the line/trapeze wire combination tight to that point and hold that point on the line tightly with your thumb and a finger. Now walk with the line to the back of the boat and again pull the line taut and mark on your hull behind the rear beam where your thumb hits, and see what number setting it is at. We will want to set the rake at 7" behind the rear beam for starters. This will allow the sailplan to generate lots of lift in the lulls. This is a great setting because it doesn't hurt in puffs near 25 mph. If the sustained wind was in the mid-20 range, then we would want to begin our mast rake at 10" behind the rear beam. If the sustained winds are upper 20's with gust in the 30's, rake all the way back (Remember to set your rig tension to your racing setting before you check your mast rake.)

Hope this helps
Jon
H20, TheMightyHobie18