I dont know exactly which was the biggest thing, but using a extender for such short time (total less than 100hrs I estimate) it's got to have been the pumping for 3 months once a day that gave me the stretched flaccid length SFL increase of 2/3 inches. Im going to incorporate the
Lengthmaster now in my routine and hope to really start seeing some EL increases, just need to buy it but will do that very soon.
And to answer how much you can gain the big studies come to the conclusion of on avarage people gained 1 inch in 1000hrs with tension around 1-1,5kg. They measure only tension and hours. However I think it's more important to measure lenght in the extender, go for more bars as soon as it is possible and repeat. patience. I see you are maybe a little but unsure on what to do and you really are motivated for gains which is great! Just pick one thing - manual lenghtmaster or extender. And do PE every single day until you have 1000hrs from either one and I can guarantee you will have 1 inch, at least.