You have to gain exp from the mobs for it to count so you can't be lvl 50 CP 160 and go to any non vet dolmens and expect the summoned Daedra you kill will grant FG for example. Doing the Dolmen itself for the first time might give a small bonus though I am unsure. AFAIK you need to be able to gain exp from the mobs in order for it to count.
Note 5 levels under your level still grants exp OP just less optimal than 5 levels above...
So you will just gain FG faster than you will gain leveling exp.
I assume this is for staying in Blackwater Blade with Dawnbreaker?
On 2 of my chars I achieved FG 10 @ around level 40-42.
On most of my others I have surpassed level 50 long before reaching 9. So I can't really say anymore as I try and get to max level ASAP so I can start equipping my chars with proper gear and start using them.
so TLDR:
If you're killing mobs 5 levels under you you will still gain exp just at a slower pace so it is prob the best way to do this...
If you're DC and in your 20's or 30's I would recommend the zombies @ the docs @ Sentinal harbour...
I once started there @ level 17 because Rivenspire grinding spots were all taken and it was a long time before IC was a thing, it was one of the chars I hit FG 10 long before 50.
