Да, дилетантски, но получилось
var blockOffset = parseInt( $('.services__list').offset().top ),
blockHeight = parseInt( $('.services__list').height() ),
windowHeight = parseInt( $(window).height() ),
blockWindow = blockHeight + windowHeight,
blockHeightOffset = parseInt( blockOffset + blockHeight ),
blockEnd = parseInt( blockHeightOffset + 0 )
halfBlock = parseInt( blockHeight / 2 ),
blockPercent1 = blockHeight / 100,
scrollTop = parseInt( $(window).scrollTop() ),
otherBlockOffset = parseInt( blockOffset - ( blockWindow - blockHeight ) )
diff = blockHeightOffset - scrollTop;
if ( scrollTop < blockHeightOffset && scrollTop > otherBlockOffset ) {
console.log( 1 / ( ( blockHeightOffset - otherBlockOffset ) / diff) );
}