There is no definitive way to measure Parkinson's. Two of the most often used methods are:
UPDRS, Unified Parkinson's Disease Rating Scale
MDS-UPDRS, Movement Disorder Society‐Sponsored Revision of the UPDRS.

(Note in both these measures low scores are good, since they show less disability.)

Each rating scale is formed from sections.

Each section is formed from questions/observations.

Each question is, usually at least, marked on a 5 point scale (0-4).

The maximum score, showing complete disability, for the UPDRS is 199 and for the MDS-UPDRS it is 272.

The first thing to do is note whether the score is "total" or some subset of the whole scale: Part III, motor scores are frequently reported.

UPDRS maximum section scores are as follows: Part I, 16 points; Part II, 52 points; Part III, 108 points; Part IV, 23.

You often read papers saying something like "the mean effect of this drug is to reduce UPDRS total score by 5 points".

Now, I think it is more likely that your personal measure of Parkinson's is made in percentage terms. For instance, you might think that your PD is 10% worse than it was a year ago.

So, to get a sense of a 5 point claim made in the literature you need to convert points score into a percentage score. Now the natural inclination is to divide 5 by 199 and multiply by 100 to get 2.51%. This isn't too impressive.

But, you make your assessment of the claim from where you are now compared with where you would be if you took the drug. So, suppose that your UPDRS score is now 50, the 5 point decrease would represent a 10% decrease. This is worth considering.

Holden et al. report the following results [1]:

"MDS‐UPDRS scores increased in a linear fashion over 5 years in patients with de novo PD. MDS‐UPDRS total scores increased an estimated 4.7 points per year, Part I scores increased 0.92 points per year, Part II scores increased 0.99 points per year, and Part III scores increased 2.4 points per year."

They show mean total MDS-UPDRS scores increasing from 31.75 at baseline, to 38.33, to 42.34, to 45.78, to 52.00, to 54.89 in the following 5 years.

Another thing to look out for is whether the results are based on medicated or non-medicated groups:

"The continuous linear time longitudinal model predicted an expected annual increase of 1.77 points ... for MDS‐UPDRS Part III for the medicated group. For the unmedicated group, Part III scores were predicted to increase by 4.02 points per year ... . There was a statistically significant difference in the slopes for MDS‐UPDRS Part III progression between the medicated and unmedicated groups, with an estimated difference of 2.25 points per year".

The MDS-UPDRS test can be seen at [2].

References:

[1] "Progression of MDS‐UPDRS Scores Over Five Years in De Novo Parkinson Disease from the Parkinson's Progression Markers Initiative Cohort"
Samantha K. Holden MD Taylor Finseth MD Stefan H. Sillau PhD Brian D. Berman MD
Movement Disorders Clinical Practice, September 2017
https://onlinelibrary.wiley.com/doi/...002/mdc3.12553

[2] https://www.movementdisorders.org/MD...lish_FINAL.pdf

John