Going from Persian (Farsi) to Khmer (ភាសាខ្មែរ) has a composite linguistic distance of 82% (heuristic estimate). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Arabic → Khmer
Indo-European ↔ Austroasiatic
The 5 languages with the lowest linguistic distance from Persian (Farsi):
A quick 3-question check between Persian (Farsi) and Khmer. No signup.