The first thing to say is that when this article was first written there was no cash out button on Betfair, so life is easier if you just want to lock in a profit and green up on a horse you've bet, no calculator required. But don't expect to get the best bet possible using the cash out button!
But you should know the maths and how to calculate your own stakes for greening up and that's what we cover on this page.
Yesterday Pete asked how do we calculate our stakes when we have made a trade and we want to green up.
So Pete has backed a horse that is shortening in price and he wants to green up on that selection. IE lock in a profit whatever the outcome.
If for example you were lucky or smart enough to get on Shaws Diamond yesterday morning you could have bet it at around 16.0.
This mare started with a Betfair SP of 6.64. (Easier to find these greening up opportunities in hindsight!!)
So let's say you had a back bet for £10 at 16.0 in the morning and it's just before the off and your horse is now trading at 6.8.
You want to lock in a profit on Shaw's Diamond and want to know what stake to lay at 6.8.
The calculation is to multiply your initial stake times your initial back odds (£10 x 16.0 = £160)
You then divide that by the lay price that you can get (£160/6.8 = £23.52)
Place your lay of £23.52 at 6.8 and your initial bet of £10 has now converted to a guaranteed payout of £12.84.
I mentioned a while back that a lot of serious backers don't wait for a selection to win these days.
Instead they lock in a profit once their horse trades down to a certain level and then lock in the profit.
This method is great for predetermined trades or for when you have time to calculate the stake.
When the pressure is on you might prefer to use lay calculator software that can make the bet in the click of a button 
If you want to it the other way around. So say you have one that's drifting and you lay a tenner at 6.8 and now can back it at 16.0.
Multiply your lay price by your lay stake (£10 x 6.8 = £68) and then divide that by your new odds (£68/16.0 = £4.25).
Back your selection for £4.25 at 16.0 and lock in a £5.46 profit.
A final note: It's easier to find steamers just listen to the pundits and check out free tipsters. Also you risk less money in the initial bet and get a better profit on the green up.
(Green Up: Common parlance for a guaranteed profit on Betfair where every selection shows it will profit if that selection win)