How Manglik is checked
A chart is Manglik when Mars occupies the 1st, 4th, 7th, 8th or 12th house counted from a reference point. The calculator counts from all three the tradition uses, the ascendant, the Moon and Venus, and reports which of them fired. Some regional lists add the 2nd house; the result says when a chart is Manglik only because of it.
The intensity grows with the number of reference points that fire. Mars from the ascendant alone is a lighter case than Mars fired from all three.
Can a Manglik marry a non-Manglik?
Often, yes. The first thing to check is whether the Manglik chart carries a cancellation: Mars in its own sign or exalted, Jupiter aspecting or joining Mars, or Mars in a Mercury sign where the dosha comes only from the 2nd house. Several traditions also hold that the dosha weakens after about 28. The result lists each one, whether it applies, and why.
Where no cancellation applies, the case deserves a proper reading of that Mars rather than a label: its sign, what aspects it, and which houses it rules for that ascendant. The sources do not say such a marriage is doomed.
Double Manglik
When both charts are Manglik the dosha is treated as neutralised. It is the most widely accepted cancellation in the whole system, and the reason Manglik is traditionally checked for both partners at once.
Manglik and the Guna Milan score
Mangal dosha adds no points to the 36 and takes none away. It is reported next to Guna Milan because the two are checked together, not because one changes the other.