If you are dutch betting i guess you are only looking at your top raters ,,you could do something like this..
Give a final rating out of 10 ,,,

Horse RTG Calculation Winning chance
Makybe Diva 7 ....... .. .7/19 X .8 29.5
Lonhro 7 ....... .7/19 X .8 29.5
Sunline 3 ....... .... .3/19 X .8 12.6
Kingston Town 2 ........ ...2/19 X .8 8.4
total 19

Makybe Diva 100/29.5 price $3.40
Lonhro 100/29.5 price $3.40
Sunlne 100/12.6 price $8.00
Kingston Town 100/8.4 price $12.00

This helped me alot when i started out,,,got it from a site somewhere ...
The .8 is of course framing the market at 80% ...taking into account the horses not listed ,,this would also be in a 10-12 horse field...i'd go alittle higher in a smaller field and smaller in a larger field..

Hope this helps,,but if you search this forum you'll find heaps of helpfull articles..