I had a 147 diagnostic and made it to PTing at 170 with a 167 on my actual test. Don't worry on starting low, your diagnostic really isn't that important, it's just a starting point. Focus on learning section by section. What I found best was working hard at something like logic games, then once I got better at that I would focus one by one on the other two sections til I was at a similar skill level at all 3 then restart the process back at logic games.
I had a 147 diagnostic and made it to PTing at 170 with a 167 on my actual test. Don't worry on starting low, your diagnostic really isn't that important, it's just a starting point. Focus on learning section by section. What I found best was working hard at something like logic games, then once I got better at that I would focus one by one on the other two sections til I was at a similar skill level at all 3 then restart the process back at logic games.