I've run lower dose deca/npp in the past. 100mg/week makes my joints feel a bit better. 200mg/week does even better and i do actually notice some strength increases here. 300 mg is the highest i've ever gone. If you want to just go for some joint love 100 mg/week should help you out some so long as your joints aren't completely screwed. The general rule of thumb you'll find as a starting point is a 2:1 ratio of test/nandrolone. So 300 test/150 npp in your case though i'd start at 100mg. You will have to inject npp either every day or every other day.

I’m still trying to understand why you are in 300mg test weekly… that would have me in a middle ground.

Add another 0.

4-600 is my sweet spot. I just don't see 50 mg doing much. Waste of $

I have some experience on this topic. So here are my anecdotes.
I've added pharma deca to my trt protocol at 70-100mg per week in the past. Doc prescribed for knee pain. it did not do anything for my joints though. I have degraded cartilage in the knees, probably too far gone. My training partner swears by 5 units of deca per day, says he feels it in the joints when he removes it.
I feel like it helped some with gym gains but no real way to quantify. My blood pressure was higher and it impacted estrogen (test deca = estrogen city). I ran extra dim and calcium d glucarate.
Subsequent experiments with npp didn't agree with me. The same side effects plus inconsistent erections.
Take that for what it's worth. I was running it at a similar dose to what you're proposing and my test dose at the time was a "sports trt" around 250-300mg per week.
Daily pins.
All that said, if you want to experiment at that low of a dose, go for it. Using a short ester, it'll be out of your system quickly if it doesn't agree with you. Nandrolone is one of the most well-studied compounds out there.

I understand what you are saying. Im just using random numbers for example but you are questioning if for example you are doing 100mg of Testosterone. Is it better to add another 100mg of Testosterone for a total of 200mgs or is there a benefit to adding 50mg of NPP for a total of 150mg. Besides volume probably not much.

Although some people do swear by microdosing tren. Which sometimes I could argue just up your Test a little more.

50mg NPP will do absolutely nothing spread across an entire week, personally I wouldn't run less than 300mg/wk in general if you want to get notable results back from it. 50mg Test E will not be noticeable either - if you want to see a notable difference there you will have to take the weekly dosage up to 500mg/wk from 300mg/wk.

Given your stats are at 6’ 175lbs with 18% bf I wouldn’t consider adding anything else to your cycle. Eat more and cleaner, check your training program so that you are making and tracking advancements and ensure you’re sleeping enough. Your goal on this cycle should be getting your weight up and bf down which won’t come from adding more gear.

50mg of npp would be kinda ridiculous to pin being you should pin a shorter ester at least 3x a week, and on top of that just too low of a dose. This question is insanely vague but yea you will more than likely see more gains above 300mg test. It depends like what you’re trying to do exactly