Is there a 'rule of thumb" regarding the percentage of mortgage interest one can claim on their taxes? Such as, if combined income of a married coujple is $80,000, federal tax deduction is approx $7,000 and there are no children to claim, would one deduct the the $400 per month in interest x 12 months from their income before determining the amount of tax due, or would they deduct a portion of $400 x 12?