Hi there,
I don’t know if you’ve already figured it out but to fine tune your speeds select the clip on your timeline, hit Apple + r, there will be a green “Retime” line above your clip. Move your mouse to the right-hand side or end of your clip, click on the green and simply drag it out/in to your specific time, i.e. drag to the left to speed up clip and shorten duration, or drag to the right to slow down your clip and lengthen duration.
Hope this helps!
Áine