I have a roll that I am doing to develop later today that I shot sunny 16 on.
So are you saying to shoot at 400 iso with the new test film and develop the same as before? Develop for 800 or develop for 400.
If you want to use HP5 at 640/800 you should expose it as such. That's the speed you were intending to use the film at when you stated your problem. You should shoot it at the same speed where you had the problem, just to prove (or disprove) that the exposure is the problem.
If you shoot it at 400, you're not addressing any of the issues you had to begin with. Troubleshooting is about taking one variable out of the scenario at a time, and otherwise do everything else exactly the same. No cheating on this. If you try to adjust more than one parameter, you will never be able to tell with certainty what went wrong to begin with, in case your actions fix the problem.
And then you end up not learning anything.
So, shoot the film at the speed where it didn't work out for you, but using Sunny 16 in broad daylight. Develop the film exactly as you did last time without changing a single thing.