# Horizontal and vertical positioning of text in thumb index

I am trying to ensure that the letters in thumb index are centered both horizontally and vertically and have a trouble adjusting the size of the thumb index or setting centering within it. Theoretically, for horizontal alignment eventxtindent and oddtxtexdent should help position the letters but the trouble is with varying width of the letters - they cannot all be LE, RO aligned and end up centered. I am trying to avoid centering each letter manually...

With vertical alignment, I am somewhat clueless. Whatever I do the letters end up slightly lowered in the thumb rectangle and I want them exactly centered.

%!TEX TS -program = xelatex
%!TEX encoding = UTF-8 Unicode

\documentclass{book}

\usepackage[paperwidth=190mm, paperheight=280mm, textwidth=170mm,  textheight=263mm,

\usepackage[width=12mm, height=9mm, distance=0.5mm, eventxtindent=7mm, oddtxtexdent=2.5mm, topthumbmargin=22mm, bottomthumbmargin=12mm]{thumbs}

\usepackage{lipsum}

\begin{document}
\pagenumbering{arabic}
\chapter{A}