Score = Σ (fit weight × attribute match) + Σ (behaviour weight × recency multiplier) − Σ (disqualifying deductions).
Fit points
Attributes of the lead and their organisation: industry, company size, role, country, technology in use. These are stable, known early, and describe whether this is the kind of customer you can serve well.
Behaviour points
Actions taken: pricing viewed, demonstration requested, email replied to, trial started. These describe whether the lead is doing anything now, and they should be capped per action type so repetition cannot dominate the total.
Recency multiplier
A factor that reduces the weight of older actions. Without it, accumulated history outranks present interest, which is the opposite of what the model is for.
Negative points
Deductions for signals that indicate a non-buyer: a free email domain where your customers use business addresses, a careers page visit, a competitor domain, a country you do not serve.
The threshold
The cut-off at which something happens. Choose it deliberately against current data, so that the qualifying band is a workable minority rather than most of the list.