Anyone knows excel and Chemistry, need help with an acid base calculator for students

Hello Everyone

It has been some time since my last post here and I thank everyone for their help.
I have been working on my pH calculator I have been able to get the acids and base calculators to work independently very accurately of each other and anyone who I am permitting use license free of the attached book.

However I need some help to get the acids and bases to work together to create a pH of an acid and base reaction, ergo I need someone to lend me a hand who is familiar with chemistry

This is where I stand

I have 500ml of 2M of H3PO4, this gives me a H+ concentration of .1188 which has a pH of .9253 Easy enough.

I have a 100ml of 0.0143M of NaOH which gives me a OH- concentration of .01339 which has a pH of 12.1458 again easy enough.

To me I should just be able to take the .1188 and subtract the .01339 which would give us an H+ excess of .1048 which would equate to a pH of .97970 however find with several calculators it reports the pH at .927- .93 which is more acidic. even IF I take into consideration the amount of water formed in the reaction which would make the solution more base than the .97970.

What am I doing wrong? the challenge is here I do not want to use the moles value of the excess i.e. of the H3PO4 as there will be other multiproteic acids like EDTA added in to calculate the pH of multiple acids and bases.

I want to be able to extend this out to use up to 24 different acids and bases, up to 12 acids and up to 12 bases. Hence WHY I prefer if possible to use the H+/OH- concentration rather than the individual chemicals and their reactions.

Can anyone tell me what I am doing wrong and could lend a hand.