That is particularly interesting to me as I own the same camera and considering buying a macro lens to give DSLR scanning a go. What particular lens do you use (AI vs AF, etc...)? I seem to remember that 55mm has a 1:2 ratio, are you able get reasonably close to the negative for 35mm work?
The way I calculate it for my 16MP Fuji APS-C mirrorless, to fill the height of my 23.6mm x 15.6mm sensor with a 36mm x 24mm film frame, the required magnification is (15.6/24 =) 0.65x magnification, or 1:1.5.
If your lens reproduces the 36x24mm film frame at 1:2 (0.50x or 50%) magnification, then the size of the image will be 18mm x 12mm, right? So if your APS-C sensor is the same size as mine, there would be (23.6-18mm =) about 5.6mm of unused sensor width, and (15.6-12mm =) 3.6mm of unused sensor height.
Calculating the areas: my APS-C is about 368 sq.mm, and the half-size film image would measure 216 sq.mm. So, at 1:2 magnification you would be using about 58-59% of your sensor's available area.
Put another way, if the image of the film frame fills the sensor of my 16MP Fuji APS-C sensor, the digital image will have dimensions of about 4896 x 3264 pixels. But at 1:2, a digital capture of the film frame will be closer to something like 3734 x 2560 pixels, or about 9.6MP (I think; someone should probably check my math.)
Only you can decide if that is "reasonably close" for your intended use of the digital files.
My advice would be to get the thinnest extension ring you can find for your 1:2 macro lens, which should give you enough magnification to fill the area of the sensor (I'm guessing).
If you want to digitize more than a few frames, it takes a lot of time - so I would want to get as much resolution as practical. Otherwise, you may wish you had more resolution later, which will require doing it all over again.