Use Count as a Condition in "Custom Column" with Power Query

Got it.  See if this works:

let
   Source = Table.Combine({MovimentosCBLPocas, MovimentosCBLSVTV}),
   MergeFinanciamentos = Table.NestedJoin(Source, {"Conta+Empresa"}, Financiamentos, 
   {"Conta+Empresa"}, "Financiamentos", JoinKind.LeftOuter),
   ExpandFinanciamentos = Table.ExpandTableColumn(MergeFinanciamentos, "Financiamentos", 
   {"Contratado", "Tipo"}, {"Financiamentos.Contratado", "Financiamentos.Tipo"}),
   Grouped = Table.Group(ExpandFinanciamentos, {"Conta+Empresa", "Financiamentos.Tipo"}, {{"Count", each Table.RowCount(_), Int64.Type}}),
   Filter = Table.SelectRows(Grouped, each ([Financiamentos.Tipo] = "Financiamentos")),
   MergeBack = Table.NestedJoin(ExpandFinanciamentos, {"Conta+Empresa", "Financiamentos.Tipo"}, Filter, {"Conta+Empresa", "Financiamentos.Tipo"}, "Counts", JoinKind.LeftOuter),
   ExpandTotals = Table.ExpandTableColumn(MergeBack, "Counts", {"Count"}, {"Count"}),
   Divide = Table.AddColumn(ExpandTotals, "Eval", each [Count] / [TotalUsed], Int64.Type),
   ReplaceInvalids = Table.ReplaceValue(Divide, each null, each [SingleValue],Replacer.ReplaceValue,{"Eval"})

in 
   ReplaceInvalids

Without seeing sample data and sample expected end result and going based solely off your initial code block, this is the best I've got.  It's close to what you had in your original post.  I think you were on the right track (assuming this solves your need).

*Fixed an oversight where I didn't do the division*