I found a trig formula on Sheldon Brown's website that uses the difference in length between the two forks and the wheelbase measurement of the bike to calculate the change in headtube angle. I used an online trail calculator for trail figures. My bike, as is, has a trail measurement of 74. Replacing my 40mm rake fork with one of 50mm yields a trail measurement of 63. However, as was noted in this thread, a longer fork will affect the heatube angle, which throws off the calculations I did. The measurements I used to calculate the change in head tube angle were off in both the difference in length of the forks and the overall wheelbase. Those numbers indicated the longer fork would decrease my headtube angle by -.78 or something like that. That new angle gave a trail measurement of 68. Given that the actual fork length differences was wrong by a factor of two and the wheelbase I used was too short, the change in fork angle should be less than -.78....I think. I figure it will be somewhere between 63 and 68 as a ballpark figure. Whether it makes a discernable change in the bike's handling remains to be seen.