The eventual goal is to assign minimum value of actl for a new exp value (where actl is missing). I think I should first come over the issue of drawing a line passing through these observations to use for a decision rule, but not really sure how to attack this problem.
Thank you in advance

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input float(actl exp) byte minactl
 .  .4306503 .
.6 .43129745 .
.4  .4318457 .
.4   .432377 .
.4  .4326184 .
.4  .4332968 .
.3  .4353066 .
.3  .4364009 .
.4  .4389387 .
 .  .4391871 .
 .  .4402585 .
.4  .4403833 .
 . .44135585 .
 .  .4416456 .
 .  .4417434 .
 .  .4419167 .
 .  .4438399 .
.4  .4453161 .
 .  .4481203 .
 .  .4515771 .
.4  .4517835 .
 .  .4522247 .
.4  .4527117 .
.3  .4529815 .
.3  .4544157 .
.5  .4677871 .
.4  .4767839 .
.6  .4869735 .
.6  .4926984 .
.4 .51891696 .
.3 .52825445 .
.4 .53248686 .
.4  .5328437 .
 .  .5332037 .
.6  .5334881 .
.6  .5335155 .
.5  .5338645 .
.5 .53483564 .
.6 .53746325 .
.5  .5404169 .
 .  .5407301 .
 . .54236794 .
.4  .5438738 .
.5  .5484574 .
.5 .55163646 .
.4 .55418557 .
 .  .5592771 .
 . .56299645 .
.8  .5669411 .
 .  .5672508 .
 .  .5673133 .
 . .56743836 .
 . .56973493 .
 .  .5727739 .
.8 .57350165 .
.6 .57357556 .
 .   .573942 .
 .  .5740153 .
 . .57531005 .
 .  .5799186 .
.6  .5843775 .
.5 .58538944 .
.6 .58968616 .
.5 .59198374 .
 . .59305274 .
.4  .5949966 .
.4  .5956155 .
.7  .5968927 .
.8  .5977873 .
.7  .5997788 .
 .  .6584147 .
 1  .6827189 .
 1  .6881836 .
 .  .6882014 .
 .  .6898127 .
 1  .6934553 .
 .  .7054474 .
.7   .707083 .
 .  .7075651 .
 .  .7090087 .
.9  .7098652 .
 .  .7100893 .
 .  .7105488 .
 .  .7108207 .
 .  .7110068 .
 .  .7116353 .
 .  .7138406 .
.6  .7139603 .
 .  .7140106 .
 .    .71864 .
 .  .7186521 .
.4  .7188953 .
.5  .7188953 .
 .  .7204999 .
.7   .720687 .
 .  .7210785 .
 .  .7215325 .
.7  .7218328 .
 .  .7218585 .
 .   .728121 .
end
enter
scatter actl exp

to see the scatter plot. Visually I am trying to get a line passing through the lowest XY points to assign the minimum value of Actl to a new Exp var that does not have a known Actl value yet.