I'd say experiment. it's dependent on the situation at your mark. Wind ( -direction ), casting weight, bait size and weight and required distance. There's no strict rules for brake blocks. But don't start with the lightest ones, better safe at first and speed up the spool later.
Cheers mate , what i meant in my post was i have seen that the brake blocks come in Small , Medium or Large sizes and which is the right size too use in a fathom 15 . Thankyou for your advice though "Flat Out" it will come in very useful .
OK, for practical i'd say. Get yourself a large and medium set. Start large and if the reel feels lifeless change for the medium ones. Forget about the small blocks, they're no use for a reel the Fathom 15's size.
Still the block size vastly depends on the fishing conditions, as mentioned. Experimenting is part of the game.
Send it to Russ at Blakdog tackle, and get it magged.
You can then adjust the reel to any situation without the ball ache that changing blocks is, especially at night on the beach when an offshore wind does a 180.
I got Shimano one's here (medium) if you're interested, @smithy73?
Ahhh, I see the old wives tale of messing around with brake blocks on a beach is still brought up as some form of argument against centrifugal control reels. I have never needed to remove or replace b/blocks with larger ones whilst fishing - nor do I see anyone else doing it. But that's me.
I tune reels specific for their application. If distance casting with large spools like sloshes and saltists, two medium size b/blocks, thin oil and moderate use of the spool tension cap to suit alternate bait sizes and weather conditions. I have explained in greater detail this moderately used spool tension cap, with pictures, in post #9 within the b/block tutorial. Not broke or replaced a thing despite the scaremongering you hear to not use it.
If the reel is going to be tossing big baits and shorter distances, big brake blocks and thicker oil is more apt.
The slower the spool revolves (lob), minimises the b/blocks braking efficiency. Longer casts generates more braking/friction.
Spool size plays a major part too when aiming for control. The larger capacity spools like 525 gs/slosh/saltist will be heavier than a Abu 6500 spool which means larger spools have more inertia/momentum when they get going. Spools with greater inertia need greater braking.
Casting over land,... turning the mag away during cast matters. Yes. But. Fishing, nåh, blocks work fine. Adjust for safe. Mag works fine. Adjust for safe. Cod, Whiting, Place, Huss, they don't really care about mags or blocks,... ;-)
And I wonder if b/blocks can generate enough frictional heat from a cast, that it's possible brake fade could occur at the end of the cast, meaning an automatic reduction in spool braking without twiddling anything.