Using Tostring in formulas

Using Tostring in formulas

I'm using Tostring in this formula, it's a String formula field, we are doing some math operations and then using Tostring. It saves and then the results on screen:

Concat(If((((1+Tonumber(${Contacts.Tasa}))^(1/12))-1)/(1-1/(1+(((1+Tonumber(${Contacts.Tasa}))^(1/12))-1))^(Tonumber(${Contacts.Plazo})*12))*Tonumber(${Contacts.Valor})/Tonumber(${Contacts.Tasa})/((${Contacts.INGRESOS}+${Contacts.Salario})*0.85+(${Contacts.Rentas}*0.7))<=0.31,'SI','NO'),' con ',Tostring((((1+Tonumber(${Contacts.Tasa}))^(1/12))-1)/(1-1/(1+(((1+Tonumber(${Contacts.Tasa}))^(1/12))-1))^(Tonumber(${Contacts.Plazo})*12))*Tonumber(${Contacts.Valor})/Tonumber(${Contacts.Tasa})/((${Contacts.INGRESOS}+${Contacts.SalarioVariable})*0.85+(${Contacts.Rentas}*0.7))))


Result on screen:
NO con 1+12.0^1/12-1/1-1/1+1+12.0^1/12-1^20.0*12*1.5E8/3000.0/10000+5000*0.85+1500*0.7

a couple other formulas using Tostring only also shows part of the math operation on screen. How can it be solved? we need the whole math operation get done before running the Tostring part.