how much mark room

That sounds sea man like to me, rule does not say racing ideal rounding!

I presume that you were windward boat throughout the rounding?

Gordon

I was in this case the windward boat

The discussion in the 2005-2009 Case 21 may be helpful

CASE 21

Rule 18 Preamble
Rule 18.2(a), Rounding and Passing Marks and Obstructions:
Overlapped – Basic Rule
Definitions, Room

The extent of the room that an outside right-of-way boat must
give at a mark or obstruction depends on the existing
conditions.

Question

What is the maximum amount of room an inside boat without right of way
is entitled to take in rounding or passing a mark or obstruction? What is the minimum amount that the outside boat is required to give?

Answer

The possible answers vary widely. To suggest the extremes, they might be:
1. as a minimum, enough room with sails and spars sheeted inboard for
the hull to clear by centimetres both the outside boat and the mark or
obstruction;
2. as a maximum, all the room the inside boat takes, setting her course as far abeam of the mark as she wishes.

Neither is correct.

As the definition Room and the preamble to rule 18 state, the word
‘room’ in rule 18 means the space needed by an inside boat, which in the
existing conditions is handled in a seamanlike way, to round or pass
promptly between the outside boat and the mark or obstruction, including
room to tack or gybe when either is a normal part of the manoeuvre.
The term ‘existing conditions’ deserves some consideration. For
example, the inside one of two dinghies approaching a mark on a placid
lake in light air will need relatively little space beyond that required for her hull and properly trimmed sails. At the other extreme, when two keel boats, on open water with steep seas, are approaching a mark that is being tossed about widely and unpredictably, the inside boat may need a full hull length of room or even more to ensure safety.

The phrase ‘in a seamanlike way’ applies to both boats. First, it
addresses the outside boat, saying that she must provide enough room so
that the inside boat need not make extraordinary or abnormal manoeuvres
to keep clear of her and the mark. It also addresses the inside boat. She is not entitled to complain of insufficient room if she fails to execute with reasonable efficiency the handling of her helm, sheets and sails during a rounding.

the bit that interests me is   'she must provide enough room so that the inside boat need not make extraordinary or abnormal manoeuvres '