Please correct me if I am wrong, but this is not about comparing two fields within the same index, but two fields within the same document?
If so, you could use a script query in the search input that subtracts high from lo and checks if the value is above a threshold. However this is not a scalable solution as this calculation needs to be done for every hit.
You could use the ingest script processor to create the difference of those two values at index time and then run a much faster query, that is doing a range query for all documents whose treshold is above X.
Hope that makes sense.
Regarding the spaces questions. Right now this is not possible.
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