Going from Estonian to Croatian/Serbian (Hrvatski) 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.
Latin → Latin/Cyrillic
Uralic ↔ Indo-European
The 5 languages with the lowest linguistic distance from Estonian:
A quick 3-question check between Estonian and Croatian/Serbian. No signup.