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)
Pat Gainer, of vit C fame, recommends solving it in pure isopropyl alcohol (1g/100cc alcohol) - no scent, colors, etc.
I use it, don't know how long it lasts, but it works fine.
In the long past I've used it with bisulphite - but I honestly do not remember how long it lasted...
So how accurate is it? Let say you decide that you can measure 2 grams easily. Dissolve the 2grams into for example 100ml of water. You now know that 100ml of this solution would give you 2grams. 50 ml would give you 1gram. 10ml would be 0.2 grams. 7.5ml would be 0.15 grams.
Now you can do the same thing with any amounts that are convient.