I predetermine at the beginning of the year what my bankroll is going to be and then allow for each " * " to be equivalent to 1% of my beginning bankroll. I am a bankroll nit though. Others may say that each " * " or unit is equal to 2% or 5% of their bankroll. Each is just a way to measure a bet.
If I bet a game at 1* then I am betting enough money to profit 1*. Therefore, if I bet a game that has a line of (-150) then I am betting 1.5 X the equivalent of 1*. If a game has a line of (+150) then I am betting 1* / 1.5.
Example: If each * = $100 then a game at a line of (-150) would result in a bet of $150 to win 1* or $100. If I were to bet a game with a line of (+150) then I would be risking $66.67 to profit $100.
Other people I have seen predetermine that each " * " is how much they are going to risk on each bet. So they will bet 1* on a game with a line of (-150) and they will profit 0.67* and with a game that has a line of (+150) they will bet 1* and profit 1.5*.
Everybody may use a slightly different variation which is most comfortable to them. I know that this explanation is not the most concise or correct description but maybe you can understand the concept that I am trying to get across even though I did not explain it all that well.