A computerized system and method process financial securities and instruments to accurately determine and optimize the after-tax proceeds an investor could expect to have at the end of a holding period for each of a set of investment strategies and determines an optimal strategy for maximizing such after-tax proceeds. The computerized system and method receive tax and investment data, user-customized investment expectations, and financial adviser-based investment expectations at a processor; perform tax loss harvesting analysis on the user-customized investment expectations and the financial adviser-based investment expectations over a dynamic taxation time range using a predetermined software program; perform comparative pro-forma tax sensitivity analysis of the tax and investment data and the analyzed investment expectations using the predetermined software program; and determine and output an optimal after-tax investment strategy path from a plurality of investment strategy paths over the dynamic taxation time range using the predetermined software program to optimize the after-tax proceeds from the plurality of investment strategies. The predetermined software program may include a spreadsheet program.