In 'theme_boost_get_pre_scss' there is the code:

$scss .= '$font-size-base: ' . (1 / 100 * $theme->settings->fontsize) . "rem !default;\n";

which assumes that the 'fontsize' setting is in 'rem' units.  However:

1. If a theme declares a setting of the same name (in effect to override the setting) with a non-numeric value then the error 'A non well formed numeric value encountered' happens when 'Purge all caches'.  This is because of the way that 'get_pre_scss_code' works in 'outputlib.php' and the child theme theme_config instance (referenced by $theme) is used with the Boost code 'theme_boost_get_pre_scss'.
2. Boost has no 'fontsize' setting delaired in its 'settings.php'!

Therefore this code is redundant and I propose it should be removed = its a bug!

Code's been there since MDL-56511 with the string:

$string['fontsize_desc'] = 'Enter a fontsize in %';

stating percent and yet the code is 'rem'!  Thus a bug in MDL-56511.