Hi all!
I asked that question on DP Review, and I was redirected here because are analog specialists.

I need to expose 4x5 and 8x10 color neg film sheets in medium-ish-gray-like density (Kodak Portra 160). I do have the backs & dark slides, but no camera. I do not want any image on there; just plain and consistent medium-ish gray all over. I need this to be repeatable, and it need to be about the same from 4x5 to 8x10.

Based on the suggestions I got on DP review, I'll add this:
-A lab will be processing these, not me.
-I know the result isn't going to be gray when looking at it, I know about and I'm ok with the orange cast of the neg base
-I may not be the most technical person, but I did learn & work in analog photography before converting to digital
-I do need to do this with a color neg, not a B&W
-I'm not looking to make a filter of any kind
-I work with the break-down processes of the negative material
-The specific density of the neg isn't that important; I'll be able to judge from the tests strips


I am unsure how to proceed, and due to the cost of these materials now a days, I thought I'd ask first!

Here are the options right now:
1- I was thinking backs facing a large softbox in a dark room and:
A- incident metering at the place of the film back. Then, take off dark slide, flash, close dark slide. Setting on light meter: Iso 160 (for an iso 160 neg film), aperture to 1, speed
to 5 sec (so it catches the whole flash duration). Then reduce light of 1 stop. Or put light meter to iso 80.
B- measured at the place of the back on a grey 18% card in reflective, same light meter settings.

2- Expose with my B&W enlarger with a 80A cc filter to correct for the 2900K light bulb. I'd take a sheet and make time-strips of .5 sec or .2 sec to decide on correct exposure.

3- Rent an 4x5 and a 8x10 view camera and point it on a white wall illuminated with flash, work in reflective and correct for bellow extension.

Any thoughts on what could work and what would not? Other, simpler suggestions?
Thanks in advance,
Lodo
PS: English isn't my first language, so please forgive my mistakes.