How would you use this %, whether it is net or gross? The only time I looked at this was to figure out what % of my salary to deduct for my 401K, and that was really just to try to even it out over the full year and still max it out. I always maxed by ESPP at the full 10%. Those deductions are from gross salary. Other than that, when I was working full time I auto-invested some amount with Vanguard, but that wasn't based on any % of income, but rather what I could afford in my budget. I also invested most of my windfalls from bonuses.
If you're looking at some guidelines that you should save x% of your income so that you can live at y% of income in retirement, you can do better than those guidelines. In the early years I'd say just sock away as much as you can and focus on a LBYM lifestyle. As you get older you can better estimate what retirement income you'll actually need, and be able to set a nest egg goal based on a safe withdrawal weight and save towards that goal.
If you're just keeping score, whatever you want to do is your business. If you want to compare to others, I'd guess most use gross, but I always figured it'd be better to worry about myself rather than what others are doing.