I taught for many years and Higgins is OK for beginners. Many students would already have it from previous classes. I would offer alternatives for them to try and most of the time, students would be thrilled with the progress they made after finding some more delicate inks. The easiest one is walnut ink.
I know from my classes with Mike Sull that he recommends McCaffrey's ink for beginners - but he probably leaves Higgins as an option )in his books) because it is so widely available. McCaffrey's is very popular with pointed pen enthusiasts. But NOT the GLOSSY black. That stuff has serious drying problems.
John Neal has a set of 10 McCaffrey's colors.
https://www.johnnealbooks.com/product/mccaffery-sample-jars-set-of-9Use them up promptly and then you have 10 dinky dips.
Walnut ink is also dreamy with pointed nibs. You can buy it by the bottle, but I prefer the crystals and to mix it myself.
Ziller inks are pretty good - and a good option if you want water*proof* lettering on envelopes. But, it is acrylic based so you do not get the finest hairlines.
If I were stuck with only Higgins eternal, I would dilute it a bit with distilled water. Remember with any ink, if you work straight from the bottle, there will be some evaporation while the lid is off. Don't dilute the whole bottle - experiment with a smaller quantity in a dinky dip - or some other small container.
McCaffrey's and walnut ink are both *watery* and some people do not like the look - they prefer super dark black. I actually like the watery look. So, maybe some others will post suggestions for really dark black alternatives.