DTI ratios confuse me

[Joe Boggin]

i'm actually not confused by the ratios I do understand how to calculate my own ratios. I have a couple friends that own their own mortgage companies and from what I gather the max ratios that lenders will lend for conventional 30 yr are around 45. I was told someone would lend up to 50 if compensating factors were adequate. 

Lenders use gross income to calculate lending power. However homeowners take-home 70% of their gross income roughly.  Am I missing something?  I know everybody gets taxed returns and write offs but it just seems like the lending structure really needs adjustments.