Does anyone know of a source regarding which finger 'daktylos' refers to (in regards to greek measurments) - the thumb / middle finger / etc.
Daktylos is the Greek word for “finger”. As a unit of measurement it designated the breadth of one finger. I think all fingers (apart from the thumb) are about the same breadth.
From the "New Pauly", Brill 2015:
Daktylos (162 words) Article Table of Contents
 Measure of length
The daktylos, Latin digitus, as a measure, is the term for the fingers' width, with four dáktyloi constituting a palm (παλαιστή; palaistḗ, Latin palmus), 16 daktyloi a foot (πούς; poús, Latin pes) and only in Greece 12 daktyloi making a span (σπιθαμή; spithamḗ). In Rome however the daktylos can also, according to the duodecimal system, be equated with the uncia and be counted up to the as (= pes). The guide for the daktylos is the foot that measures between 29.4 and 35.4 cm. It therefore fluctuates between 1.84 and 2.21 cm. Smaller distances are measured in fractions of the daktylos. Square and cubic daktylos were not in use.
Measures; Palaiste; Palmus; Pes; Pous; Spithame
Mlasowsky, Alexander (Hannover)
F. Hultsch, Griech. und röm. Metrologie, 21882, 28f., 74f.
O. A. W. Dilke, Digit measures on a slab in the British Museum, in: The Antiquaries Journal 68, 1988, 290-294.
The daxtylos is based, like much of early Greek science, on Egyptian standards. A finger is the distance between the tip of index finger and the crease of the first joint. In most people, this distance is close to one inch.
A palm is 4 fingers. A hand is 5 fingers.
So, to answer your exact question: it is the index finger's first joint. You can find a discussion of this topic in "Ancient Egyptian Construction and Architecture" by Clarke and Engelbach.
(Note that many reference books and sources incorrectly refer to a dactylos as being a "finger's breadth". In fact, it is absolutely clear that in both Greece and Egypt 4 fingers make one palm, and this would be impossible if a finger's breadth were meant, because a finger's breadth is less than a 1/4th of a palm. It is, in fact, the length of the first phalange of the index finger, which is exactly 1/4 of a palm.)