I just buy as is needed. I work on the assumption that if I'm buying lots it's because we're doing lots - and assuming all my prices are based on my costs and outgoings etc, profit will follow.
However, I don't buy automatically, so I constantly monitor what's doing well and not so well. If we finish a shellac was it one we used a lot or was it a shelf sitter that's not worth replacing?
On the months with less spend, I simply spend less and any profit carries over towards the next month.
One advantage in our business is that we're not waiting to be paid - like builders for example. We know what we've got!
I sometimes find it helps to transfer money to another account so I have a 'bank' of money. I do this on a weekly basis. The same amount each week. I transfer an amount every Friday which pays my rent, but I transfer quite a bit more than my rent. Into the same account goes my lessees payments so this soon adds up. It gives my a bit of a cushion or I use it to pay myself a dividend.
Cash flow is always hard but I really rein it in and don't spend until I have to. The fact that Ellisons do next day delivery is a godsend.
Vic x