D’oh that’s what I get for trusting my mediocre Hebrew keyboarding skills. For those who don’t know the Hebrew alphabet, both ת and ט are commonly transcribed as T (the former often as TH, but it’s meant to be a German aspirated T sound rather than the English th).
אבגדהוזחט - count one through nine
יכלמנסעפצ - count ten through ninety
קרשת - count one hundred through four hundred
so תו would be 406 while טו is 9+6=15