Since PercentRelativeLayout was deprecated in 26.0.0 and nested layouts like LinearLayout inside RelativeLayout have a negative impact on performance (Understanding the performance benefits of ConstraintLayout) the best option for you to achieve percentage width is to replace your RelativeLayout with ConstraintLayout.
This can be solved in two ways.
SOLUTION #1 Using guidelines with percentage offset
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/host_label"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Host"
android:layout_marginTop="16dp"
android:layout_marginLeft="8dp"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintLeft_toLeftOf="@+id/host_input" />
<TextView
android:id="@+id/port_label"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Port"
android:layout_marginTop="16dp"
android:layout_marginLeft="8dp"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintLeft_toLeftOf="@+id/port_input" />
<EditText
android:id="@+id/host_input"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:inputType="textEmailAddress"
app:layout_constraintTop_toBottomOf="@+id/host_label"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toLeftOf="@+id/guideline" />
<EditText
android:id="@+id/port_input"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:inputType="number"
app:layout_constraintTop_toBottomOf="@+id/port_label"
app:layout_constraintLeft_toLeftOf="@+id/guideline"
app:layout_constraintRight_toRightOf="parent" />
<android.support.constraint.Guideline
android:id="@+id/guideline"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
app:layout_constraintGuide_percent="0.8" />
</android.support.constraint.ConstraintLayout>
SOLUTION #2 Using chain with weighted width for EditText
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/host_label"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Host"
android:layout_marginTop="16dp"
android:layout_marginLeft="8dp"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintLeft_toLeftOf="@+id/host_input" />
<TextView
android:id="@+id/port_label"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Port"
android:layout_marginTop="16dp"
android:layout_marginLeft="8dp"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintLeft_toLeftOf="@+id/port_input" />
<EditText
android:id="@+id/host_input"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:inputType="textEmailAddress"
app:layout_constraintHorizontal_weight="0.8"
app:layout_constraintTop_toBottomOf="@+id/host_label"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toLeftOf="@+id/port_input" />
<EditText
android:id="@+id/port_input"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:inputType="number"
app:layout_constraintHorizontal_weight="0.2"
app:layout_constraintTop_toBottomOf="@+id/port_label"
app:layout_constraintLeft_toRightOf="@+id/host_input"
app:layout_constraintRight_toRightOf="parent" />
</android.support.constraint.ConstraintLayout>
In both cases, you get something like this