Vision is typically reported as xxx/yyy where the xxx value is usually 20 for US assessments. As vision gets worse, for the same numerator, the denominator increases.
logMAR is log10(denominator/numerator) or -log10(numerator/denominator)
"normal" vision is 20/20, or logMAR = 0
20/100 is worse than 20/20 and logMAR = 0.69897
So the logMAR increases as vision gets worse and decreases as vision gets better
if you are doing change = visit - baseline, a negative change would be improvement in vision
a positive change would be worsening in vision.
Another interpretation of change in logMar values is to take the antilog of the change in logMAR values - this would be the "number of lines" in which vision changed. FDA often applies a 3 lines of change (ETDRS chart) criteria as this change is a doubling of the visual angle.
A useful reference on calculating average visual acuity and the whole logMAR concept is the article by Jack Holladay "Proper Method for Calculating Average Visual Acuity".
It is also useful to refer to FDA Guidelines for Multifocal Intraocular Lens IDE Studies and PMAs and Guidance for Industry Guidance for Premarket Submissions of Orthokeratology Rigid Gas Permeable Contact Lenses.