Have a look in The Darkroom Cookbook (steve anchell) p28:

water at 125f/52c 750 ml

Sodium bisulfite 6g

Phenidone 2g

Cold water to make 1000ml

100 ml of concentrate contains 0.2g of Phenidone . I've used this formula and it works a treat. Keeping qualities reasonable as long as you keep it cool (& try Tetenal Protectan spray)