Strength of any house is determined on the basis of three criteris :
Placement of planets in the house, which is most important normally, but if the planets are debilitated and the lord of that house is strong, then strength of the lord becomed more important in priority,
Strength of the house-lord :
Main strength depends of following factors in this order : exaltation, moolatrikona, svagrigi, adhi-mitra-grihi, mitra-grihi, sama-grihi, shatru-grihi, ati-shatru-grihi, neecha, which have respective strengths as follows : 60, 45, 30, 22, 15, 8, 4, 2, 0 (Ishta-kashta-vivechana-adhyaaya, BPHS, verses 7-9). These numbers are not to be compared with Shadbala numbers. Thus, we see Exaltation-to-Debilitation has a very high gradient, while Shadbala has a low gradient of two times approximately. Moreover, two successive categories in exaltation-debilitation grading have a gradient of about two times (logarothmic scaling with base two, as in modern Information Theory, used in BPHS !). Therefore, Shadbala is a subset of this Main Strength, and Shadbala should be used ONLY when Main Strength of two competing planets same Main Strength.
Moreover, if Lord has no aspect on its house, its influence will decrease but will never be zero. My experience is that zero aspect by Lord is equivalent to 50% normal aspect (I have coputed it from rainfall data).
Aspect of Lord is more important than other planets.
But RELATIONS modify strengths greatly, hence must be taken into account. For instance, if some planet is conjunct with the Lord, its aspect will be more important than that of another planets which are unrelated to the house in question or its lord.
Vakri planet is more strong according to BPHS. An auspicious Vakri becomes more auspicious, and an inauspicious Vakri becomes more inauspicious.