Don't think you can remove the sum symbols as you are using Ninox built in function.

RE warning.... you propabbly could add a conditional style 

ie click in the column header and add a rule!

add something like >= (your value ie 300.00) then you can slect the text colour/scell colour etc to change

ie in my example

anything above 90% changes the text colour

However - I am not sure it will show when the summary figures are collapesd and you may need to expend them?

oops loaded 2nd image a bit small !

Nope - don't think it works on the summed totals
just tried adding a one off formula too- but no good (it works but only wnen you expand the totals)

Seems Ninox ignores conditional styling on summarized totals. Probably because they are virtual and don't really exist in the table.

I see you have a dashboard. Why don't you use that to get your summary?

One thing I have learned is that interacting with your data through a dashboard is the way to go.