Here is a text from Uttara Kalamrita about retrogression.

वक्री स्वोच्चबलः सवक्रिसहिते मध्य बंद तुञ्गभे
वक्री नीचबलः स्वनीचभवने वक्रीबलं तुञ्गजम्।
उच्चस्थेन युतोर्द्धवीर्यमिति चेन्निचे तु सुन्यं बलं
मितैःपापखगैः शुभैरिपुखगैर्युकतोपि चार्धं बलम्।।

Reads

vakrī svocca-balaḥ savakra sahite madhyaṁ balaṁ tuṅgabhe
vakrī nīcabalaḥ svanīcabhavane vakrībalaṁ tuṅgajam

uccasthe na yuto ‘rddha vīryam iti cen nīce tu śunyaṁ balaṁ
mitraiḥ papakhagaiḥ śubhai ripukhager yukto ‘pi cārdhaṁ balam

Building A Translation

A) vakrī svocca-balaḥ savakra sahite madhyaṁ balaṁ tuṅgabhe

1. Regtrogresser gains exaltation-strength if in its exaltation sign.
2. A planet with a regressor gains half that, too.

B) vakrī nīcabalaḥ svanīcabhavane vakrībalaṁ tuṅgajam.

Retrogressor with debilitation-strength in its debilitation-place, gains the retrogression-strength gained from exaltation.

C) uccasthe na yuto ‘rddha vīryam iti cen nīce tu śunyaṁ balaṁ

1. “With one not in exaltation-place, half-strength”
2. But in debilitation-place, null-strength.

D) mitraiḥ papakhagaiḥ śubhai ripukhager yukto ‘pi cārdhaṁ balam

With friendly malefics and benefic enemies, half-strength.

Translation

A retrograde planet gains a ṣaḍ-bala called “exaltation-strength” (uccha-bala). A planet in its vicinity can also positively impacted, and strengthened by as much as half a point.

A planet in its debilitation sign gains no exaltation-strength, but still gains the full retrogression-strength, which is equivalent.

Exaltation strength is in-between if not in exaltation, and null if in debilitation.

Proximity to friends and enemies, benefics and malefics, also gives proportional strength.

Explanation

One category of strength for a planet is “exaltation strength.” The author says an exalted planet gets 100% of a point (“rūpa”), a debilitated planet gets 0%, and those in between get the in-between value. (Typically it is calculated by degree distance to the exaltation and debilitation degrees, not signs. So the “in-between” value is the ratio proportional to the proximity to these degrees).

Retrogression also affects this category. It can add to the exaltation strength. If a planet is retrograde it gains an additional full-point.

Conjunction also affects this category. Conjunction with a retrogressive planet gives an additional half-point. Conjunction with friends and enemies, malefics and benefics also adds, as follows.

1. If the conjunction is with a planet that is beneficial and friendly, give that full value (i.e. half-point).
2. If it is benefic but unfriendly, give half it (quarter, total)
3. If it is malefic but friendly, also give half.
4. If it is malefic and unfriendly, give nothing.

In the end, the ucca-kalamrita version of uccha-bāla (exaltation strength) will be a range from 0 to 2.5 or potentially higher if there are multiple conjunctions.

Notes: this requires using inherent friendships, not situational. Also, Line D may be referring to the friendliness and beneficence of the retrogressive conjunction, not to any and all conjunctions. It is not clear.