Fix battery scaling computation to account for buy rate in export

What's Changed

Changes to the metric computation

Previous Predbat valued the current battery level at the maximum of the minimum import value or the minimum export value. This is now changed to account that if you export you will have to import again and thus the true value maybe lower. The change was made as on Intelligent go evening export sometimes wasn't planned.

• Fix battery scaling computation to account for buy rate in export by @springfall2008 in #1130

Full Changelog: v7.19.6...v7.19.7