Why do you suspect that yours and the other one you found are inaccurate?
There's really nothing fancy. 4 for an A, 3.67 for an A-, 3.33 for a B+, 3 for a B, and so on. Weight the grades by multiplying by the number of credit hours, and including all courses taken across all institutions, add up the sum of the weighted grades. Divide by total credit hours to get overall GPA. Include only biology/chemistry/physics/math to get BCPM/sGPA. If you're using a calculator that more or less does this, then you should get an accurate answer. The hardest part is making sure you're including all courses, including dual credit/community college courses. Otherwise it's just a tedious matter of inputting all courses