I remember that question from a "friend" in math class, we were like 14/15. he asked :" when you jerk off , does it splashes everywhere or does it dribble on your hand "?
I said something like " when I think about you it dribbles but when I think about her ( the girl behind us) it splashes everywhere "
I disagree with the correlation of amount of sperm and length you can shoot.
you can starve your dick for days and just cum few drops at 10 cm. You can also jerk off a second time 5 minutes after the first shot and go very far. Remember that although it's true spermatozoids are created 24/24 and stocked in the epididymis, it only represents 20% of the total volume. (remember also if spermatozoids are not used for an extend period of time ( I know I learned that but can't remember how long they can be stocked) they are 'dismantled' and by-products are reused to create new one. so if you dont have an ejaculation for 10 years , it's not gonna be more than for 10 months of abstinence). 60% of the volume is the result of the secretion from the seminal vesicle.
when you fuck you usually (not always, sure) go further than alone (unless you are being watched on cam). that's basic psychology : when someone is looking at you , you are in an unconscious state of competition to mate. You sperm needs to be the first to reach the ovum that's why you need to reach the end of the vagina faster ( they are experiment about this , absolutely fascinating, even the count of sperm , the traction is influenced by other guys . They tested that with rats around one female).
Also when I was like 16 and bored of homework I was mentally trying to reach mental targets. and it works. if you visualize your cum going were you want it to it's gonna help. Like you are jerking off with a friend and you want to cum on him coz.. you know it's fun
well you have to visualize your cum going up to his face for instance, and it's really going to help a lot to achieve this mischievous plan.
Now if you take steroids or any natural plant that increase your testosterone level , your amount of "sperm" is going to increase : see that guy from Bel ami (cant remember his name sorry) A roid user: he can spit loads of very transparent cum : that's because of course the seminal vesicle is activated by testosterone and so it produces a huge amount of clear liquid. (Im not encouraging you to do that just explaining).
Finally being aroused a lot, or not so much is also important as well as edging. if you are tired etc... So it's really multifactorial. (Im not even talking about the dopamine level which triggers the orgasm and that can be influenced by your general heatlh, medecine, food . some porn dude press under their balls before cumming to increase pressure ( it just gives more excitation )