A Responsive Double occurs if the opponents open and raise a suit AND partner between overcalls or doubles.
If partner doubles, an rDouble shows equal length in two suits.

If partner overcalls, an rDouble shows the other two suits.
11 2 X shows hearts and clubs
1 2 3 X shows spades and diamonds

1 X 3 X shows equal length in the majors
if you had one major longer than the other, you should bid it
1 X 3 X shows equal length in the minors
if you had 4+ of the major, you should bid it

How about points?
The higher the opponent's bids, the higher the rDoubler suggests partner bid.
The lowest rDouble auctions are:
1 X 2 X or
1 12 X
Might either rDouble above be made with as few as 6 points and four cards in each of the two suits?
With 5-5 shape, perhaps even fewer points may be exceptable. How about just 4 points?

Many play rDoubles this high:
1 X 4 X showing the minors with points.
Perhaps 12+ points is a standard for such a bid.
Primary Doubler's passing Secondary Doubler's Double for penalty is an option: xxxx AK Axxx xxx

rDoubler can't double for penalty xx QJT9 Axx xxxx, needing partner to REOPEN with a double:
Perhaps partner has a strong and shapely takeout: AKxx - AQxx Kxxxx and this is your auction:
1 X 4 P
P   X   P   P
P

You and partner might like to discuss their premption and your minimum point/distribution requirements.

Where do you draw the lines, when they open and raise in the range from 2 to 4 ?
Are your responsive double requirements for partner's overcalls different than for partner's doubles?
They should be:
1 1 requires only 8 HCP
1 X requires on average at least 11HCP
1 2requires at least 11HCP(?)
4 X requires...

How about when they preempt and raise?
2 X 4 X
3 X 4 X
Sure, why not?
Of course the requirements for a double of 3 should be higher than a double of 2 .
Shouldn't the requirements for an rDouble then be LOWER if partner has promised more?

Consider how much you need to 'survive' partner bidding one of your suits.