some more RTP advice

api/docs/v2/parameters/choosing.rst

@@ -1,5 +1,7 @@
 :og:description: Advice on choosing effective parameters in Opentrons Python protocols.
 
+.. _good-rtps:
+
 ************************
 Choosing Good Parameters
 ************************
@@ -10,3 +12,10 @@ The first decision you need to make when adding parameters to your protocol is "
 2. **Work efficiently.** Don't burden run setup with too many choices or confusing options.
 3. **Avoid errors.** Ensure that every combination of parameters produces an analyzable, runnable protocol.
 
+Consider what scientific task is at the heart of your protocol, and build parameters that contribute to, rather than diverge from it.
+
+For example, it makes sense to add a parameter for number of samples to a protocol for a particular DNA prep reagent kit. But it wouldn't make sense to add a parameter for *which reagent kit* to use for DNA prep. That kind of parameter would affect so many aspects of the protocol that it would make more sense to maintain two protocols, one for each kit.
+
+Also consider how a small number of parameters can combine to produce many useful outputs. Take the serial dilution task from the :ref:`tutorial` as an example. We could add just three parameters to it: number of dilutions, dilution factor, and number of rows. Now that single protocol can produce a whole plate that gradually dilutes, a 2×4 grid that rapidly dilutes, and *thousands* of other combinations.
+
+The trick to choosing good parameters is reasoning through the choices the protocol's users may make. If any of them lead to nonsensical outcomes or errors, adjust the parameters — or how your protocol :ref:`uses parameter values <using-rtp>` — to avoid those situations.