I don't use your same testing method, so I cannot comment on your times. I get good prints with my equipment using a gradient around 0.75 and printing on multigrade paper.

I can say that if exposure is having an effect on the gradient there is a major problem in your method; like the values of your stepwedge are not evenly spaced or not evenly illuminated.

If you are not interested in calibrating your system for 'speed' then a contact print of the step wedge may be less problematic for you.