misreckon
verbUK /mɪsˈɹɛkən/
Etymology From mis- + reckon.
1
To add (something) up incorrectly, make a wrong calculation of (an amount etc.).
2
Obsolete To make a wrong calculation, to be in error.
- And therefore he that hath once misreckoned himselfe is never seene againe., Folio Society, 2006, vol.1, p.207