I've recently purchased a GT i-drive Race (frame size: large). The problem I have is with the suspension tuning. According to the manual, in order to get the best result, one should "dot" the "i", using the 155 mm plastic caliper provided. The Fox Float RC shock already measures 155 mm, without me sitting on the bike, so I can't get the ideal length because there's always sag when I sit on the bike, even if I pump it to 300 psi. I was wondering if I should change the shock, perhaps a Rock Shox SID XC?.