Setting the General Space Between the Letter Groups in an Index with \Indexspace

Question

Update In light of David Carlisle's very helpful comment providing a simple, efficient solution to setting the space between letter groups in an idxlayout index, I have modified the title a bit to better reflect the original question. Although David Carlisle's comment responds quite nicely to the question posed, I would still like to learn, if possible, (in reference to Ulrike Fischer's remark in another post as later noted) how the objective might be accomplished with \indexspace.

Consider the code

\documentclass{book}
\usepackage{imakeidx}
\let\cleardoublepage\clearpage
\makeindex
\usepackage[]{idxlayout}
\begin{document}
\LARGE
Some words.\index{HEADING1! Subheading! Index entry1.}
Some words.\index{HEADING2! Subheading! Index entry2.}
Some more words.\index{NEXT SET OF HEADINGS1! Subheading! Index entry1.}
Some more words.\index{NEXT SET OF HEADINGS2! Subheading! Index entry2.}
\idxlayout{columns=1}
\printindex
\end{document}

which produces the index

As one can see, there is not much space between the "H" groupings and the "N" groupings. I would like to, as simply as possible, increase the vertical space between the letter groupings.

Trying to find a way that may have already been prescribed on this site, I came across the post, Vertical spacing in index with idxlayout in which Ulrike Fischer declares "The general space between the letter groups are set by \indexspace which you could redefine if you want"; but the answer given pursues an alternative route.

QUESTION: How may one specify the said space using \indexspace? Is it as simple a matter as declaring something like "\indexspace=20pt"? If so, how\where? I have not found an answer yet.

(No texindy please.)

Thank you.