A 50% margin would be half as much again (i.e 1.5 times the CP), a 100% margin would be doubling the cost price (i.e 2 times the CP) and 150% would be one and a half times (i.e. 2.5 times the CP)
Margins have more than one use. Since it is related to the Mark-up (i.e. a Mark-up of 100% will generate a Margin of 50%) Margins are a useful check on the business. If the SP is calculated to generate a 50% Margin, but the Profit and Loss is showing a Margin different from 50% then something unforseen is causing that and needs to be investigated.
If you want a Margin of say 70%. divide the CP by 1-0.70, i.e.CP/ 0.30. Alternatively make the CP 1, and add 1 to the answer to give you a Mark-up factor to multiply by. Thus 1/1-0.70 would give you 3.33, add 1 to give 4.33 and this is the Mark-up factor that will give you a 70% Margin (i.e. CP times 4.33 will give a 70% Margin) and so on.
This works both ways, to add the VAT - multiply, or to remove the VAT - divide. This gives you the VAT exclusive price. If you need to work out the VAT content of an inclusive price, DO NOT multiply by -20% see below.
This method gives you the actual VAT content of a VAT inclusive price.