I think that you're correct using common terminology. The government takes in money as taxes, and creates money through printing:

spending = taxes + printed money new money supply = old money supply + printed money

But, I believe that MMT is saying that taxes and spending could be logically separated by assuming that all taxes go to the shredder and all spending comes from newly printed money:

spending = printed money new money supply = old money supply + (spending - taxes)

So it wouldn't be 'increasing spending by taxes', it would be 'reduce inflation by taxes'. The size of 'big-gov' should be measured as spending/GDP regardless of the actual tax rate. The tax rate would float depending on the current inflation target instead of on the current federal spending.