[Tex/LaTex] Using whole page with minipages

Question

I'm preparing template for dynamic content. Content will be printent in blocks, as presented here:

Page format a4, no margins. Special printer provided. There are 21 blocks. Each block size: 70mm x 42 mm.

I'm struggling with 7th row with keeps missing. I'm using minipage with size given above for each block. The fact that 7th row keeps missing tells that there is at least one vertical spacing on the page. I set all spacing according to:
https://www.sharelatex.com/learn/Page_size_and_margins
at 0 size (top margin set to -1in) but there is no result.

Any idea what's missing ?
Or is there any other way to use 7x42mm = 294mm of page's 297mm space ?

Answer 1

Here is a primitive model to be served as a starting point.

\documentclass{article}
\usepackage[a4paper,margin=0in,top=1.5mm,bottom=1.5mm]{geometry}
\usepackage{tikz}
\begin{document}
  \begin{tikzpicture}[remember picture,overlay]
    \foreach \x in {1.5,43.5,...,295.5}{
     \draw[very thick,blue] ([yshift=-\x mm]current page.north west) -- ([yshift=-\x mm]current page.north east);
     }
     \foreach \x in {0,70,140,210}{
     \draw[very thick,blue] ([shift={(\x mm,-1.5mm)}]current page.north west) -- ([shift={(\x mm,1.5mm)}]current page.south west);
     }
\node[scale=10,text=blue!70!black,font=\sffamily] at (current page.center) {21};)
  \end{tikzpicture}
\end{document}