What do you mean by fatigue percents?

Calm shit down, build shit back up

The Fellowship of the Ring

Pick the modality you like and start somewhere manageable. Build the habit, Prioritise doing the thing consistently above all else. Nothing you do in the first year of starting cardio will be impressive to anyone anyway so don't sweat it. Prioritise avoiding injury over pushing the limits and training as hard as you could. If you get pain, drop the per session volume or intensity. GLHF Enjoy being fitter than all your weightlifter mates.

Start by teaching the client the movements with a load they can completely control. Could be a broom stick or an unloaded barbell. If they do it to your satisfaction take a small jump in weight and repeat. Repeat this process until the set becomes sufficiently challenging for them. There's no need to have a new/newer client max out or hit failure on a compound lift. If you do want them to max out for some reason you can have them hit a rep max like a 5 or 10 RM, so that the absolute load will be lower whilst keeping effort high. Don't worry about prescribing percentages in the period before you client has a 1RM, just go off perceived set difficulty (your perception, and theirs). Once your client has had a few months of exposure to the lift you've taught them, you can teach them how to safely fail the lift, e.g. using safeties or bailing on a back squat. If you're satisfied with the client's competence & that they can fail a lift safely, you can now have them max out.