First of all, some definitions. Let's say you are a serious bettor. You must have a bankroll, or Asset, or simply "all the money you are ready to lose on betting". This must exist in real world, right? And indicator ROI does not include it in it's formula, which make no sense. ROI actually 'assumes' your bankroll is indefinite. Here is an example, let say a tipster you follow has -10% ROI, and you followed him (or her) with 10$ stake per bet (flat staking). How much did you lose? Well, it obviously depends on the number of picks he published. If it's a 100, you invested 1000$, and lost a 100$. But if he published 1000 picks, you lost a 1000$ with same math, so the question is, how much money did you have at the beginning of the story, in another words, what was your bankroll. If it was less than a 1000$, you lost it all.

Now let's transfer to ROA (in some literature is called ROC, as RoCapital, as well). And we will talk in terms of units, just to be more general. So first think you have to do is decide of how many units is your asset (or bankroll made). Less units means more risk, but grater and faster profits. I recommend 20 units bankroll (so 5% of your original asset will be invested in every pick - flat staking). If a tipster is consistent, with low deviation, you can even have less units. If a tipster is deviating highly (e.g. has +20 on 50 picks, and than -20 on next 50 picks), 50 units is a better and less risky fit. 

So, what does RAO tell you? It tells you how much of your original asset or investment have you returned or profited. For example, if you use 20 units bankroll, and your current profit is +5 units, your ROA is 5/20=25%. 100% means you doubled your money, and -100% of course, you lost it all.

Hopefully I covered it all, any additional questions or explanations are welcome in comments area, to make it more clear and understandable to everyone.