Band Behind Back

I recentally bought some bands, just to feel them out I used one band wraped round my back for my speed benches, I did this for 3 weeks and last week I just used strait weight 145, 60% of my last meet bench (I also hit a new pr on the bench at 315) So with that background here are my questions:
How much tention does one (purple or light) band have? 20-30 lbs or so?

And Should I use 190 (60% of 315) for my bench cycle now? It seems kind of heavy for speed work. I was thinking I should use 170 or so just to keep the speed up.
Thanks in advance

You answered your own question - bar speed should dictate the weight, not percentages, IMO.

The light band is between 30-50 lbs.

How exactly does the speed bench with the band around your back work? Do you wrap it around your back or do you just wrap it underneath the bench?

This is from the “Accomodating resistance” artical
Bench Press, bands behind back

With this movement you perform the standard bench press but you’ll be holding a elastic band behind your back.

This creates a incredible amount of tension at the top of the lift. You’ll want to hold the top (end position) of the lift for a peak contraction before beginning the following rep.