This Isotope plugin provides a modified version of the Masonry layout mode that is compatible with percentage-sized columns.
It was designed for Isotope v1, and I have no information regarding compatibility with v2.
Just include this script after the Isotope script, then use it as a layout mode:
$blocksContainer.isotope({
layoutMode: 'sloppyMasonry',
itemSelector: '.block-class'
});
Licensed under the MIT License.