What is the the inocula/steril soil ration most effective for Infection Unit Density as an Index of Infection Potential of Arbuscular Mycorrhizal Fungi?

I do have limited amount of field soil. I can not do the serial dilutions for the Infection Unit Density. Is it 20% (200 ml of field soil inoculated in 1000 ml of steril backgound:sand soil) a good ratio? With 100 plants. Should I try 40% and 50 replicates. Thanks for your recomendation.