I used 25 E cans for a total of 500 extra energy a day instead of FHC, but when I read the +30% speed book I averaged 6m gain/day.
Had to chain in-between so I only ended off with 150m speed in the month, but if you spend it all in the gym you could probably see 200m gains in a month with FHC, maybe just a little less.
I was gaining 8-10M/day on STR+30% book using FHC and STR/SPD specialist gym. Everything really depends on which book, what stat, what perks you have making it better, and if you optimize etc. Minus overdoses obviously.
Example the cans book like what BORK mentioned above. If you use max size cans (30E), max can booster in faction (45E), and use the book (90E), you will get an additional 1080E per 24H of booster cooldown.
If you are working in a restaurant during that you can use 16 cans instead of the typical 12 per 24H of cooldown, which means an additional 1440E per 24H.
I used the cans book with maximum cans in a restaurant, I gained 250M total stats from it. My STR+30% book I was on the past month I ended gaining 230M total stats using max FHC.
Best advice for any gym book is do your research on how to maximize for that specific book (strip clubs for DEF/DEX, etc etc), any generic "do this or that for all books" is prone to not being the most efficient.
I was planning on this. I have Ugly energy and the 30% gains for def, dex, and str I wanna use in 2020. Full FHCs I forgot about the strip club tho I was thinking a FC. Thanks man <3