Instead of setting width in % on right column you can just use flex: 1 and it will take rest of free width in a row.
.flex_container {
display: flex;
width: 100%;
}
.flex_item_left {
width: 200px;
background: lightgreen;
}
.flex_item_right {
flex: 1;
background: lightblue;
}<div class="flex_container">
<div class="flex_item_left">
.....
</div>
<div class="flex_item_right">
.....
</div>
<div>