Control is pushed from the screen - xaml

Question

I have a xamarin.forms app. A screen has many controls, and I use ScrollView to let the user ability to see all the controls. I have a button outside of the ScrollView, because I want it to be easily accessible without scrolling down the screen. But some of our customers have vision issues, so they enlarge the text size on their phones. Then the button disappears from the screen. I wonder what I can do to fix this... I guess I would agree with the scrolling only for those with the large text size, if there is no better solution.
Here is my code:
<ContentPage.Content>
<StackLayout>
<ScrollView>
<StackLayout Margin="30">
... Many controls are here
</StackLayout>
</ScrollView>
<StackLayout
x:Name="Validation"
HeightRequest="150"
IsVisible="{Binding ValidationResult.IsValid, Converter={converters:InverseBoolConverter}}">
<Label Text="Please fix the following errors:" TextColor="Red" Margin="0,0,0,10" />
<Label Text="{Binding Errors}" TextColor="Red" />
</StackLayout>
<Button
Text="Calculate"
Command="{Binding CalculateCommand}"
Style="{StaticResource ButtonStyle}"
HorizontalOptions="CenterAndExpand" />
</StackLayout>
</ContentPage.Content>
ADDED:
I did not mention that the content of the page is within a ControlTemplate:
<ControlTemplate x:Key="MainPageTemplate">
<Grid>
<Label Text="{Binding ErrorMessage}" TextColor="Red" />
<ContentPresenter Margin="10, 0, 10, 120" />
<StackLayout VerticalOptions="End" BackgroundColor="LightGray" Padding="20" >
<Label Text="{Binding Source={x:Static sys:DateTime.Now}, StringFormat='© {0:yyyy} Company Name, Inc.'}" />
<Label Text="All trademarks shown are the intellectual properties of their respective owners." />
</StackLayout>
</Grid>
</ControlTemplate>

Answer 1

Agree with #Jason , StackLayout will not fit the size of its child Elements . So you could use Grid with three Rows
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="0.9*" />
<RowDefinition Height="150" />
<RowDefinition Height="0.1*" />
</Grid.RowDefinitions>
<ScrollView Grid.Row="0" Orientation="Both">
<StackLayout Margin="30" BackgroundColor="LightBlue" HeightRequest="300">
</StackLayout>
</ScrollView>
<Grid HeightRequest = "150" Grid.Row="1">
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Label Grid.Row="0" Text="Please fix the following errors:" TextColor="Red" Margin="0,0,0,10" />
<Label Grid.Row="1" Text="{Binding Errors}" TextColor="Red" />
</Grid>
<Button
Grid.Row="2"
Text="Calculate"
HorizontalOptions="CenterAndExpand" />
</Grid>