Very interesting question posted Christopher-Joseph . i dont think , we have yet quantified the optimum load of soil  bacteria and fungi in relation to production levels or any other soil parameters..?

Fungal:bacterial dominance of growth, biomass, and residue indicate different, and not directly relatable aspects, of the microbial community’s influence on soil functioning. Considering relationships to environmental factors, we found that shifts in fungal:bacterial dominance were not always in line with the general expectation, in many instances even being opposite to them. This is likely because the traits expected to differentiate bacteria from fungi are often not distinct. Considering the

impact of fungal:bacterial dominance on ecosystem function, we similarly found that expectations were not always upheld and this too could be due to trait overlap between these two groups ( Source :  Strickland and Rousk 2010, Soil Biology and Biochemistry, 42: 1385-1395) . Find one interesting PDf relating this issue. 

Please have a look at these links and PDF attachments.

http://www.farmersweekly.co.za/article.aspx?id=46650&h=The-fungal-bacterial-ratio-for-soil-health

https://extension.illinois.edu/soil/SoilBiology/fw&soilhealth.htm

https://ecologicalprocesses.springeropen.com/articles/10.1186/2192-1709-1-6

Good Luck

Dr.Antoun,well explained and I endorse the comments.

because of the many and different factors involved for both species, there simply can not be an "optimum" ration!

Is it optimal to have a higher fungal : bacterial ratio in soil, or vice versa (for healthy soil)?

In our studies in fruit crops , bacterial population we find , much higher than fungal population in any healthy orchrd....what happens in replant disease issue..??