I think, the topic should not be how accurate they are but it should be how close they are in terms of accuracy.
- A Muhurta is considered to be 48 mins = 2880 secs.
MBH 12:231.11-13:
“Vyasa said, ‘Only Brahman, which is without beginning and without end, unborn, blazing with effulgence, above decay, immutable, indestructible, inconceivable, and transcending knowledge, exists before the Creation. The Rishis, measuring time, have named particular portions by particular names. Five and ten winks of the eye make what is called a Kashtha. Thirty Kashthas would make what is called a Kala.Thirty Kalas, with the tenth part of a Kala added, make what is known as a Muhurta. Thirty Muhurtas make up one day and night. Thirty days and nights are called a month, and twelve months are called a year.
Again, Manu Smriti 1.64.:
Ten and eight ‘nimeṣas’ (should be known as) one ‘Kāṣṭhā’; thirty such (Kāṣṭhās) one ‘Kalā’; thirty ‘Kalās’ one ‘muhūrta’; and as many ‘muhūrtas’ one ‘Ahorātra’ (Day and Night).
The difference can be seen in the nimesas. Where one says 15 and other says 18.
As mentioned in question only a blink can varie from 100 milli secs (0.1 s) to 400 milli secs (0.4secs).
So, if we reverse calculate it keeping the muhurta 48 mins. In case of 15 blinks, 1 blink came out to be around 0.2134 secs and for 18 blinks it's around 0.178 secs.
I guess that's not bad. Actually, it's pretty close and good.
Now, Let's see another one.
Shiva Purana 2.1.10.:
21-22. In the case of all living beings, Brahmā, Viṣṇu, Harā, Gandharvas, serpents, Rākṣasas, etc., twenty one thousand six hundred respirations constitute the period of one day and one night, O foremost among Devas.
23-24. Six respirations constitute the period of time one Pala. Sixty such Palas constitute one Ghaṭī. Sixty Ghaṭīs constitute one day and one night. (6 x 60 x 60 = 21600). There is no limit to the number of respirations of Sadāśiva. Hence He is undecaying.
And, interestingly these 21600 breaths of a whole day if converted mathematically it becomes 24 hours in modern terms. See below.:
Note.: s = seconds, b = breaths, p = palas, g = ghatis, m = minutes, h = hours, d = day.
Pala (पल).— Unit of time equivalent to 24 seconds.
So, 1b = 24s ÷ 6b = 4s.
Nos. of seconds in a day as per as scriptures = 4s × 6b × 60p × 60g = 86,400s.
Nos. of seconds in 24 hours = 60s × 60m × 24h = 86,400s.
Therefore, 21,600b = 24h = 1d.
You can also check this answer of mine for more.
I hope this helps. Prd..