OK, here's a simple example for entering a height as either feet and inches or as cm.
.height-in-feet
= f.input :height_in_feet, required: true, input_html: { :"data-mfl-hint-target" => "div.height-details", :"data-mfl-validate-proxy" => "input#consumers_details_health_height", maxlength: 1 }
.height-in-inches
= f.input :height_in_inches_after_feet, required: true, input_html: { :"data-mfl-hint-target" => "div.height-details", :"data-mfl-validate-proxy" => "input#consumers_details_health_height", maxlength: 2 }
.height-in-centimetres
= f.input :height_in_centimetres, required: true, input_html: { :"data-mfl-hint-target" => "div.height-details", :"data-mfl-validate-proxy" => "input#consumers_details_health_height", maxlength: 3 }
.hidden