Retirement tax questions

I have this same issue and have in the past, but didn't pay much attention to it..   Really trying to determine if I am leaving money on the table so to speak.  

 

When I do publication 560...   

 

Net Business Profit =  2,032     

SE Tax deduction = $144   

Adjusted Net Profit = $1,888

 

Using 20% for max allowed from employer for contribution I get $378 that I Can contribute to SOLO 401K from business profits. 

 

Turbo Tax is telling me that number is $1,888.      Am I missing something with taking the 20% ??   I thought that was all that was allowed. 

 

My understanding is that in my case with the numbers I have the SOLO 401K and SEP IRA from business profits are the same amount.   Correct??

 

Any advice would be appreciated.  

 

Thanks!