Here is how I tend to use risps in my teaching currently.
I offer this more as a springboard for the imagination than as a strict formula.

I explain to my students at the start of the year
that I want them to be prepared to think hard about a fresh problem at the start of each lesson.
I won't give them much initial help with these:
they must be ready to be creative self-starters from the word go.

My students often come in to my classroom to find a risp up already.
The risp needs to be clear enough to enable students to get going straight away.
I will give some words of welcome and make sure that everyone is getting their mind into gear.

After the initial administration, I start by visiting those I currently consider to be the the weaker students.
Are there words they do not understand? Are there misconceptions emerging? (If so, good!)
Can they see the point of the question? (If not, the risp needs to be refined.)
Ideally the risp creates an environment where students can freely discuss what they do and do not understand without feeling threatened.

Now it is time for those I currently see as the the stronger students.
By now they should have negotiated the start of the risp with relative ease.
Now is the time to challenge them with the tougher end of the risp, and to ask them to go beyond it.

Let me assume you have a full range of 'ability' in your class.
If you divide the group into those you see as likely to get A-B grades,
those likely to get C-D grades and those likely to get E-U grades, a rough rule of thumb is this:

The first third of the risp should be immediately accessible to all three groups.