Your number example is right in certain case: You work entirely in RGB color system, not with physical paints and "mix 1:1 red and white" means you watch the red color through 50% transparent white.
In that case your HSL(0,50,50) is equal with RGB(191,64,64). When that's seen through a half-transparent layer of white RGB(255,255,255), the resulted color is RGB(223,160,160). Each number is got as average.
If we convert it back to HSL, we get HSL(0,50,75) as you assumed. The lightness (=L) is the average of the largest and smallest RGB number divided by 255. In this case L=((223+160)/2)/255 = 0,75 = 75 %
Someone could shout "it cannot be as saturated as the original after inserting white!!!!" This is natural for Photohop users, because there the saturation would be smaller. But Photoshop uses HSB (also known as HSV) where the saturation has totally different meaning.
You can find conversion formulas here https://www.rapidtables.com/convert/color/rgb-to-hsl.html
With a little elementary math you get the next general formula for the new HSL lightness after mixing with white:
L2 = P+(1-P)L1
L1 is the lightness of the original color, P is the opacity of the white in scale 0...1. In your example case L2 = 0,5+(1-0,5)0,5 = 0,75 as proposed.
Do NOT expect as simple formula for cases where neither of the mixed colors is white nor other grayshade.
Can this be used with real paints? Answer: I cannot see how it could work. HSL assumes RGB screen, paint has no red, green nor blue leds, it reflects ambient light. Paint manufacturers have published paint mixing tables and curves which are valid only for their listed paints. The listed mixing results are shown as printed cards or as curves or numbers in some non-screen dependent color system like CIELAB.
I haven't seen any usable general math formulas for the color of mixed paints. I guess it's caused by the complexity of the interaction possibilities between paint material molecules in the mixed paint itself and the resulted new interaction possibilities between light and paint.