Putting a value on lot size

[Khaled El Dorry]

Hello,

I have a property under contract in the 1st ward of Houston.  There are a 4 houses that sold for very attractive $$$.  They are all in the $1200-$1600 sqft range.  The house I have under contract is 730 sqft but the lot is only 1950 sqft.  I plan to bring the property up to 1200 ish sqft to compare with the other properties size wise but I dont know how much to discount the lot value?  I used an arbitrary $150K discount but I am wondering if I am leaving money on the table (or being delusional). 

Should I use the tax value on the land and subtract the difference or are there different methods to value this so I can get equal sqft/sqft analysis to the other properties discounted for the lot?

Any words of wisdom on this would be greatly appreciated?

The other property lots are 5000 sqft so quite a bit of difference from the the lot I have. 

Would be happy to discuss more detailed $$$ in a PM settings.