fdo#77319: set spinbox value AFTER adjusting no. of decimal places

Doing it the other way round truncates the values

Change-Id: Ieb14c9d1b9dde496eac24737aced7c8da8ce1346
Reviewed-on: https://gerrit.libreoffice.org/11865
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>