Would you imagine if I get the exposure correct on a digital SLR it'd relate closely to what is on film?
Yes, and my advice is to set your camera in its most 'vivid' mode and the use the histogram to judge whether your exposure is correct. DSLRs make lovely meters
I see no reason to routinely rate velvia 50 at 40. How would you translate your dslr metering results to ISO 40 film anyway, that's a math headache

Stick to 50 for now until you see compelling reasons not to do so. Do not overexpose: if anything I'd go in the opposite direction and rate it at 65.... but obviously this depends on your own [hopefully consistent] metering practices and what you're after. It's pointless for us to debate how to rate the film if we all have different metering practices and moreover different objectives with regards to colour.
If you are a total newbie to slide then I would say bracket with one frame rated at 50 and the next at 65. On a lightbox you will prefer the latter, whereas for scanning you will probably prefer the former. Indeed, how you expose should be determined in part by how your final output will be generated.
Note that if you really need more shadow detail, you're probably better off preflashing than overexposing your slide. Overexposing is very seldom a good thing for slide. It's the opposite of print film: underexposing print film is also usually a bad idea because you'll lose shadow detail. If you overexpose slide film and you lose highlight detail, and you lose it
very quickly.
My quick 'n dirty approach is to decide what in the scene needs to be rendered white and let everything else fall where it may. This quite often means simple average metering with a small bias toward protecting highlights.