Fixed or Variable Liability?

[Speculator_3]

Some time ago, I started experimenting with trading matches using live pictures, in the vein of Psychoff et al.
I mainly look at the game flow, but I also sometimes use the recent performances of the team as a guide to help me make my decisions. So far I've done around 100 trades or so, and I am in profit.

My liability, most of the time, tends to be of fixed size for every trade I make (around 4% of my total bank). But with some trades I use lower liability (depending on whether I deem the chances of me making a profit from a trade are smaller than usual - around 2% of my bank).

However, looking over my profit/loss statement, I notice that my overall profit would be quite a bit higher if I actually used fixed liability ALL of the time (essentially this comes from me actually profiting from the trades that I thought I have smaller chances than normal of pulling off).

My question is, at this stage when I am still "trialling" a strategy, would using a fixed liability yield more information in the long-term as to how successful my strategy is? I think the added difficulty here is that this is obviously not a fixed strategy - I could be backing / laying different markets at different odds at different times in the match, depending on how the game flows. What is the general guideline for this?

Further, with this kind of approach using live pictures, just how many matches would one expect to trade to be able to gauge effectively, beyond reasonable doubt, if one genuinely has the knack of "beating the market odds"? Are we talking hundreds or thousands of matches?