Hi Tenacious,
In another thread I have posted http://forums.ozmium.com.au/showthread.php?t=7676
a spreadsheet that will provide a Statistical Weighting to each Factor ranking, provided you keep records for each Factor your using in your Handicapping you can use my spreadsheet to create a Probability line by using either of two methods from that sheet, i'll detail one using the ROE indicator, as the other is complex but a lot more precise.

Method A) Using the ROE indicator (from the spreadsheet), this is called the Ratio of Observed to Expected.
1. Compute the ROE figure for each ranked runner for each Factor.
2. For each runner MULTIPLY the ROE for each Factor together and then MULTIPLY the final total by 10.
3. This is the Individual Ability rating for each runner.
4. TOTAL all the runners Individual Ability ratings
5. DIVIDE each runners Ind. Ability rating by the TOTAL
6. These are your Projected Probabilities

If you don't have records nor win rates for each Factor you could look at INDEXING each Factor using 60 as the minimum score. an 100 as the highest rating, then computing much along the lines as above