vbox badness helvetica 12pt and 1.5 spacing

My university asks for 1.5 spacings, and at least 2cm left and right. I’d like to use arial and a header as stated below.

Now, I get vbox badness and I read this means pages are badly defined.

But what is the proper page layout? How do I figure that out? So what do I have to set bottom and/or top in the geometry package, so everything is properly shown?

documentclass[12pt,twoside,a4paper]{book}
usepackage[utf8]{inputenc}
usepackage[english]{babel}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}
usepackage{csquotes}


%% use arial
usepackage{helvet}
renewcommand{familydefault}{sfdefault}

%% use 1.5 spacing
linespread{1.5}

%%%%%%%%%%%%%%%%%%

%% Outer layout
usepackage[a4paper,width=160mm,top=25mm,bottom=35mm]{geometry}
usepackage{fancyhdr}
pagestyle{fancy}

% redefining chapter numbers
makeatletter
def@makechapterhead#1{%
  vspace*{20p@}%
  {parindent z@ raggedright normalfont
    ifnum c@secnumdepth >m@ne
      if@mainmatter
        %hugebfseries @chapappspace thechapter
        Hugebfseries thechapter.space%
        %parnobreak
        %vskip 20p@
      fi
    fi
    interlinepenalty@M
    Huge bfseries #1parnobreak
    vskip 40p@
  }}
makeatother


% in the book class, the first page of a chapter uses the plain style. Hence, redefining the plain style with fancyhdr
fancypagestyle{plain}{%
fancyhead{}
fancyhead[LO]{leftmark}
fancyhead[RE]{rightmark}
renewcommand{headrulewidth}{0.4pt}
fancyfoot{}
fancyfoot[RO,LE]{thepage}}

fancyhead{}
fancyhead[LO]{leftmark}
fancyhead[RE]{rightmark}
renewcommand{headrulewidth}{0.4pt}
renewcommand{chaptermark}[1]{markboth{MakeUppercase{#1}}{}} %makes Chapter 1 dissapear in header
setlength{headheight}{15pt} % this adjusts header higth since otherwise headersize too small

fancyfoot{}
fancyfoot[RO,LE]{thepage}

(For example, setting bottom to 15mm doesnt show a badness but that doesnt look good)