Let's say you want to make 1/2 MS formula.
If you buy just MS salts without organics, then you simply cut it in half and add enough sucrose to make 20-30g/L in the final concentration, and agar for 6g/L.
If you are using MS packets that already contain sugar and agar, then the calculation must take into account how much sugar and agar is already in the packet. You only add back in the balance needed to get to the desired final concentration.
Lets say I have a packet of MS with sugar and agar, with an assay of (say) 10 g/L sucrose and 8g/L agar (I just made these numbers up - check with your particular media for your own numbers).
If I cut this packet in half, and add each half to a liter of water, then I'll have exactly the 1/2 MS salts that I want, but the sugar/agar will be weak. In fact, I'll have (per liter):
1/2 MS (just what I want)
5g/L sugar (1/2 the rated value)
4g/L agar (1/2 the rated value)
But I really need:
20g/L sugar
6g/L agar
So I would mix in (20-5) = 15g *extra* sugar and (6-4) 2g *extra* agar to each of my two batches.
Actually, I find it convenient to break my packets up into 500ml quantities.
In this case, I'd break up the initial mix into four equal parts and put the portions in dry, brown glass bottles. To each bottle I would add 7.5g sugar and 1g agar. Store in the freezer compartment, and reconstitute with 500ml boiling water.
Nice and convenient.